Journal articles on the topic 'Clock'
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1
Son, Kyou Jung, and Tae Gyu Chang. "Distributed Nodes-Based Collaborative Sustaining of Precision Clock Synchronization upon Master Clock Failure in IEEE 1588 System." Sensors 20, no. 20 (October 13, 2020): 5784. http://dx.doi.org/10.3390/s20205784.
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This paper proposes a distributed nodes-based clock synchronization method to sustain sub-microsecond precision synchronization of slave clocks upon master clock failure in IEEE 1588 PTP (precision time protocol) system. The sustaining is achieved by synchronizing the slave clocks to the estimated reference clock which is obtained from the analysis of distributed slave clocks. The proposed method consists of two clock correction functions (i.e., a self-correction and a collaborative correction, respectively). Upon master failure, the self-correction estimates a clock correction value based on the clock model which is constructed during normal PTP operation. The collaborative correction is performed in the preselected management node. The management node estimates a reference clock by collecting and analyzing clock information gathered from the other slave clocks. The performance of the proposed method is simulated by computer to show its usefulness. It is confirmed that the fifty (50) clock model-based collaborative correction maintains 10−6 second PTP accuracy for 10 min prolonged period after the master failure when tested with clock offset variations less than 50 ppm.
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Boldbaatar, Enkhtuvshin, Donald Grant, Suelynn Choy, Safoora Zaminpardaz, and Lucas Holden. "Evaluating Optical Clock Performance for GNSS Positioning." Sensors 23, no. 13 (June 28, 2023): 5998. http://dx.doi.org/10.3390/s23135998.
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Atomic clocks are highly precise timing devices used in numerous Positioning, Navigation, and Timing (PNT) applications on the ground and in outer space. In recent years, however, more precise timing solutions based on optical technology have been introduced as current technology capabilities advance. State-of-the-art optical clocks—predicted to be the next level of their predecessor atomic clocks—have achieved ultimate uncertainty of 1 × 10−18 and beyond, which exceeds the best atomic clock’s performance by two orders of magnitude. Hence, the successful development of optical clocks has drawn significant attention in academia and industry to exploit many more opportunities. This paper first provides an overview of the emerging optical clock technology, its current development, and characteristics, followed by a clock stability analysis of some of the successfully developed optical clocks against current Global Navigation Satellite System (GNSS) satellite clocks to discuss the optical clock potentiality in GNSS positioning. The overlapping Allan Deviation (ADEV) method is applied to estimate the satellite clock stability from International GNSS Service (IGS) clock products, whereas the optical clock details are sourced from the existing literature. The findings are (a) the optical clocks are more stable than that of atomic clocks onboard GNSS satellites, though they may require further technological maturity to meet spacecraft payload requirements, and (b) in GNSS positioning, optical clocks could potentially offer less than a 1 mm range error (clock-related) in 30 s and at least 10 times better timing performance after 900 s in contrast to the Galileo satellite atomic clocks—which is determined in this study as the most stable GNSS atomic clock type used in satellite positioning.
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Reddy, M. Praveen Kumar, K. Ashwin Kumar, S. Rajesh Kumar, and RA K. Saravanaguru. "Improvement of Physical Clock Synchronization Algorithm by Two-Level Synchronization." INTERNATIONAL JOURNAL OF COMPUTERS & TECHNOLOGY 11, no. 6 (November 5, 2013): 2648–52. http://dx.doi.org/10.24297/ijct.v11i6.3039.
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Synchronization of the clocks is one of the essential thing for many applications in distributed systems. Clock synchronization is very important because they improve the performance and reliability of distributed systems. The main purpose of clock synchronization algorithms is to provide the common time to essential parts of the distributed systems. In this paper the problem considered is synchronization of clock with bounded clock drift and proposing a two level synchronization algorithm which synchronizes the processors local clocks by combining both internal and external clock synchronization.
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Robu, Andrei D., Christoph Salge, Chrystopher L. Nehaniv, and Daniel Polani. "Measuring Time with Minimal Clocks." Artificial Life 25, no. 4 (November 2019): 383–409. http://dx.doi.org/10.1162/artl_a_00303.
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Being able to measure time, whether directly or indirectly, is a significant advantage for an organism. It allows for timely reaction to regular or predicted events, reducing the pressure for fast processing of sensory input. Thus, clocks are ubiquitous in biology. In the present article, we consider minimal abstract pure clocks in different configurations and investigate their characteristic dynamics. We are especially interested in optimally time-resolving clocks. Among these, we find fundamentally diametral clock characteristics, such as oscillatory behavior for purely local time measurement or decay-based clocks measuring time periods on a scale global to the problem. We include also sets of independent clocks ( clock bags), sequential cascades of clocks, and composite clocks with controlled dependence. Clock cascades show a condensation effect, and the composite clock shows various regimes of markedly different dynamics.
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Kumar, Arun, Mireia Vaca-Dempere, Thomas Mortimer, Oleg Deryagin, Jacob G. Smith, Paul Petrus, Kevin B. Koronowski, et al. "Brain-muscle communication prevents muscle aging by maintaining daily physiology." Science 384, no. 6695 (May 3, 2024): 563–72. http://dx.doi.org/10.1126/science.adj8533.
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A molecular clock network is crucial for daily physiology and maintaining organismal health. We examined the interactions and importance of intratissue clock networks in muscle tissue maintenance. In arrhythmic mice showing premature aging, we created a basic clock module involving a central and a peripheral (muscle) clock. Reconstituting the brain-muscle clock network is sufficient to preserve fundamental daily homeostatic functions and prevent premature muscle aging. However, achieving whole muscle physiology requires contributions from other peripheral clocks. Mechanistically, the muscle peripheral clock acts as a gatekeeper, selectively suppressing detrimental signals from the central clock while integrating important muscle homeostatic functions. Our research reveals the interplay between the central and peripheral clocks in daily muscle function and underscores the impact of eating patterns on these interactions.
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Liu, Mochi, Yu Chen, Qian Xu, Yuzhuo Wang, Yuan Gao, and Aimin Zhang. "Mirror Clock: A Strategy for Identifying Atomic Clock Frequency Jumps." Sensors 22, no. 22 (November 21, 2022): 8995. http://dx.doi.org/10.3390/s22228995.
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Atomic clock frequency jumps directly influence the accuracy and reliability of timekeeping systems. The necessary corrections are typically implemented by postprocessing mutual comparison data between multiple atomic clocks based on the overly strict assumption that these atomic clocks are independent of each other. This paper describes the concept of a mirror clock, which enables atomic clock frequency jumps to be identified in real time without any assumptions. By comparing whether the real measured data and a corresponding mirror clock prediction fall within a confidence interval determined by the uncertainty of past physical clock data, atomic clock frequency jumps can be effectively identified and corrected. The results of several experiments using three hydrogen masers verify that the precision and recall of simultaneous jump identification reach 96.41% and 73.49%, respectively.
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Shakhmantsir, Iryna, and Amita Sehgal. "Splicing the Clock to Maintain and Entrain Circadian Rhythms." Journal of Biological Rhythms 34, no. 6 (August 7, 2019): 584–95. http://dx.doi.org/10.1177/0748730419868136.
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Circadian clocks drive daily rhythms of physiology and behavior in multiple organisms and synchronize these rhythms to environmental cycles of light and temperature. The basic mechanism of the clock consists of a transcription-translation feedback loop, in which key clock proteins negatively regulate their own transcription. Although much of the focus with respect to clock mechanisms has been on the regulation of transcription and on the stability and activity of clock proteins, it is clear that other regulatory processes also have to be involved to explain aspects of clock function. Here, we review the role of alternative splicing in circadian clocks. Starting with a discussion of the Drosophila clock and then extending to other major circadian model systems, we describe how the control of alternative splicing enables organisms to maintain their circadian clocks as well as to respond to environmental inputs, in particular to temperature changes.
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Harper, Ross E. F., Maite Ogueta, Peter Dayan, Ralf Stanewsky, and Joerg T. Albert. "Light Dominates Peripheral Circadian Oscillations in Drosophila melanogaster During Sensory Conflict." Journal of Biological Rhythms 32, no. 5 (September 13, 2017): 423–32. http://dx.doi.org/10.1177/0748730417724250.
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In Drosophila, as in other animals, the circadian clock is a singular entity in name and concept only. In reality, clock functions emerge from multiple processes and anatomical substrates. One distinction has conventionally been made between a central clock (in the brain) and peripheral clocks (e.g., in the gut and the eyes). Both types of clock generate robust circadian oscillations, which do not require external input. Furthermore, the phases of these oscillations remain exquisitely sensitive to specific environmental cues, such as the daily changes of light and temperature. When these cues conflict with one another, the central clock displays complex forms of sensory integration; how peripheral clocks respond to conflicting input is unclear. We therefore explored the effects of light and temperature misalignments on peripheral clocks. We show that under conflict, peripheral clocks preferentially synchronize to the light stimulus. This photic dominance requires the presence of the circadian photoreceptor, Cryptochrome.
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Markworth, Kimberly A. "Cloud Clock." Teaching Children Mathematics 24, no. 2 (October 2017): 74–77. http://dx.doi.org/10.5951/teacchilmath.24.2.0074.
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The analog clock is a rich source of tasks associated with angles and angle measures. The Cloud Clock problem is an opportunity for students to deepen their understanding of analog clocks, angles, and time and angle measurement.
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Cheng, Peng, Wenbin Shen, Xiao Sun, Chenghui Cai, Kuangchao Wu, and Ziyu Shen. "Measuring Height Difference Using Two-Way Satellite Time and Frequency Transfer." Remote Sensing 14, no. 3 (January 18, 2022): 451. http://dx.doi.org/10.3390/rs14030451.
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According to general relativity theory (GRT), the clock at a position with lower geopotential ticks slower than an identical one at a position with higher geopotential. Here, we provide a geopotential comparison using a non-transportable hydrogen clock and a transportable hydrogen clock for altitude transmission based on the two-way satellite time and frequency transfer (TWSTFT) technique. First, we set one hydrogen clock on the fifth floor and another hydrogen clock on the ground floor, with their height difference of 22.8 m measured by tape, and compared the time difference between these two clocks by TWSTFT for 13 days. Then, we set both clocks on the ground floor and compared the time difference between the two clocks for seven days for zero-baseline calibration (synchronization). Based on the measured time difference between the two clocks at different floors, we obtained the height difference 28.0 ± 5.4 m, which coincides well with the tape-measured result. This experiment provides a method of height propagation using precise clocks based on the TWSTFT technique.
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Maltsev, Victor A., and Edward G. Lakatta. "Synergism of coupled subsarcolemmal Ca2+ clocks and sarcolemmal voltage clocks confers robust and flexible pacemaker function in a novel pacemaker cell model." American Journal of Physiology-Heart and Circulatory Physiology 296, no. 3 (March 2009): H594—H615. http://dx.doi.org/10.1152/ajpheart.01118.2008.
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Recent experimental studies have demonstrated that sinoatrial node cells (SANC) generate spontaneous, rhythmic, local subsarcolemmal Ca2+ releases (Ca2+ clock), which occur during late diastolic depolarization (DD) and interact with the classic sarcolemmal voltage oscillator (membrane clock) by activating Na+-Ca2+ exchanger current ( INCX). This and other interactions between clocks, however, are not captured by existing essentially membrane-delimited cardiac pacemaker cell numerical models. Using wide-scale parametric analysis of classic formulations of membrane clock and Ca2+ cycling, we have constructed and initially explored a prototype rabbit SANC model featuring both clocks. Our coupled oscillator system exhibits greater robustness and flexibility than membrane clock operating alone. Rhythmic spontaneous Ca2+ releases of sarcoplasmic reticulum (SR)-based Ca2+ clock ignite rhythmic action potentials via late DD INCX over much broader ranges of membrane clock parameters [e.g., L-type Ca2+ current ( ICaL) and/or hyperpolarization-activated (“funny”) current ( If) conductances]. The system Ca2+ clock includes SR and sarcolemmal Ca2+ fluxes, which optimize cell Ca2+ balance to increase amplitudes of both SR Ca2+ release and late DD INCX as SR Ca2+ pumping rate increases, resulting in a broad pacemaker rate modulation (1.8–4.6 Hz). In contrast, the rate modulation range via membrane clock parameters is substantially smaller when Ca2+ clock is unchanged or lacking. When Ca2+ clock is disabled, the system parametric space for fail-safe SANC operation considerably shrinks: without rhythmic late DD INCX ignition signals membrane clock substantially slows, becomes dysrhythmic, or halts. In conclusion, the Ca2+ clock is a new critical dimension in SANC function. A synergism of the coupled function of Ca2+ and membrane clocks confers fail-safe SANC operation at greatly varying rates.
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Li, Shuaichen, Chong Li, Jianfeng Wu, and Haibo Cui. "Test and Analysis of Timekeeping Performance of Atomic Clock." Sensors 22, no. 24 (December 15, 2022): 9886. http://dx.doi.org/10.3390/s22249886.
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At present, there are few articles about the timekeeping performance of domestic atomic clocks in their moving state. In this paper, the frequency stability changes of hydrogen atomic and cesium atomic clocks in stationary and moving states are compared and analyzed; the frequency stability of the atomic clock at the beginning of its transition from moving state to stationary state is tested and analyzed; the influence of three main noises of atomic clocks on frequency stability is analyzed; and finally, the difference in the predictability of atomic clocks in moving and stationary states is analyzed. The results show that: (1) in the moving state, the frequency stability of a hydrogen clock decreases by 1–2 orders of magnitude, and the frequency stability of a cesium clock decreases by 0.5 orders of magnitude; (2) in the recovery stage, the frequency stability of hydrogen and cesium clocks is between that in static and moving stages, but the frequency stability fluctuates greatly in this stage; (3) in the moving state, the three main noises of the atomic clock all increase, of which the increase in the white noise of phase modulation is the largest, indicating that it is the most sensitive to vibration and has the greatest impact on the frequency stability of the atomic clock during the moving period; 4) in the mobile state, the RMS of the prediction data of the hydrogen clock and cesium clock greatly increases compared with that in the static state.
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Lv, Yifei, Tao Geng, Qile Zhao, and Jingnan Liu. "Characteristics of BeiDou-3 Experimental Satellite Clocks." Remote Sensing 10, no. 11 (November 22, 2018): 1847. http://dx.doi.org/10.3390/rs10111847.
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The characteristics of the improved Atomic Frequency Standard (AFS) operated on the latest BeiDou-3 experimental satellites are analyzed from day-of-year (DOY) 254 to 281, of the year 2017, considering the following three aspects: stability, periodicity, and prediction precision. The two-step method of Precise Orbit Determination (POD) is used to obtain the precise clock offsets. We presented the stability of such new clocks and studied the influence of the uneven distribution of the ground stations on the stability performance of the clock. The results show that the orbit influence on the Medium Earth Orbit (MEO) clock offsets is the largest of three satellite types, especially from 3 × 10 3 s to 8.64 × 10 4 s. Considering this orbit influence, the analysis shows that the Passive Hydrogen Maser (PHM) clock carried on C32 is approximately 2.6 × 10 − 14 at an interval of 10 4 , and has the best stability for any averaging intervals among the BeiDou satellite clocks, which currently achieves a level comparable to that of the PHM clock of Galileo, and the rubidium (Rb) clocks of Global Positioning System (GPS) Block IIF. The stability of the improved Rb AFS on BeiDou-3 is also superior to that of BeiDou-2 from 3 × 10 2 s to 3 × 10 3 s, and comparable to that of Rb AFS on the Galileo. Moreover, the periodicity of the PHM clock and the improved Rb clock are presented. For the PHM clock, the amplitudes are obviously reduced, while the new Rb clocks did not show a visible improvement, which will need further analysis in the future. As expected, the precision of the short-term clock prediction is improved because of the better characteristics of AFS. The Root Mean Square (RMS) of 1-h clock prediction is less than 0.16 ns.
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Ben-Ya’acov, Uri. "Time measurement with accelerating light-clocks." Journal of Physics: Conference Series 2482, no. 1 (May 1, 2023): 012009. http://dx.doi.org/10.1088/1742-6596/2482/1/012009.
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Abstract The clock hypothesis in relativity states that the rate of time as measured by any clock is determined by its Minkowskian proper-time, regardless of the nature of its motion; in particular independent of its acceleration, depending only on its instantaneous velocity. However, a unique proper-time may be assigned to an accelerating clock, as to any physical system, only in the limit of being point-like. But clocks, by their very nature, must be spatially extended systems, to allow an internal periodical mechanism. Therefore the question, How does the internal structure of the clock affect the clock hypothesis? The simplest model to examine the clock hypothesis is the so-called ‘light clock’, consisting of two mirrors with a light signal reflected between them. So far, such examinations were carried out mainly in the limits of point-like clocks and/or constant acceleration. Here the clock hypothesis is theoretically examined for spatially extended linearly accelerated light-clocks, parallel and vertical relative to the direction of motion, with arbitrarily varying accelerations. Using the rapidity of the clock as its evolution parameter, a Lorentz covariant analysis is neatly performed. Taking into account the spatial extension of the clock, differences between externally measured Minkowskian proper-times and the time-scale determined by the internal periodical mechanism of the clock are computed. Although these differences are practically very minute – of order aL/c 2 for characteristic acceleration a and spatial dimension L of the clock – theoretically they cannot be ignored. They indicate inherent inconsistency between the externally measured proper-time – the physical time-line which consists of moments-of-time – and the intrinsically defined age – the internal time-line, consisting of durations, intervals-of-time.
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Bothwell, Tobias. "Prospective Optical Lattice Clocks in Neutral Atoms with Hyperfine Structure." Atoms 12, no. 3 (March 5, 2024): 14. http://dx.doi.org/10.3390/atoms12030014.
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Optical lattice clocks combine the accuracy and stability required for next-generation frequency standards. At the heart of these clocks are carefully engineered optical lattices tuned to a wavelength where the differential AC Stark shift between ground and excited states vanishes—the so called ‘magic’ wavelength. To date, only alkaline-earth-like atoms utilizing clock transitions with total electronic angular momentum J=0 have successfully realized these magic wavelength optical lattices at the level necessary for state-of-the-art clock operation. In this article, we discuss two additional types of clock transitions utilizing states with J≠0, leveraging hyperfine structure to satisfy the necessary requirements for controlling lattice-induced light shifts. We propose realizing (i) clock transitions between same-parity clock states with total angular momentum F=0 and (ii) M1/E2 clock transitions between a state with F=0 and a second state with J=1/2, mF=0. We present atomic species which fulfill these requirements before giving a detailed discussion of both manganese and copper, demonstrating how these transitions provide the necessary suppression of fine structure-induced vector and tensor lattice light shifts for clock operations. Such realization of alternative optical lattice clocks promises to provide a rich variety of new atomic species for neutral atom clock operation, with applications from many-body physics to searches for new physics.
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Yu, Xinguo, Wu Song, Xiaopan Lyu, Bin He, and Nan Ye. "Reading Both Single and Multiple Digital Video Clocks Using Context-Aware Pixel Periodicity and Deep Learning." International Journal of Digital Crime and Forensics 12, no. 2 (April 2020): 21–39. http://dx.doi.org/10.4018/ijdcf.2020040102.
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This article presents an algorithm for reading both single and multiple digital video clocks by using a context-aware pixel periodicity method and a deep learning technique. Reading digital video clocks in real time is a very challenging problem. The first challenge is the clock digit localization. The existing pixel periodicity is not applicable to localizing multiple second-digit places. This article proposes a context-aware pixel periodicity method to identify the second-pixels of each clock. The second challenge is clock-digit recognition. For this task, the algorithms based a domain knowledge and deep learning technique is proposed to recognize clock digits. The proposed algorithm is better than the existing best one in two aspects. The first one is that it can read not only single digit video clock but also multiple digit video clocks. The other is that it requires a short length of a video clip. The experimental results show that the proposed algorithm can achieve 100% of accuracy in both localization and recognition for both single and multiple clocks.
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ZIĘTALA, Michał. "STABILITY OF GPS AND GLONASS ONBOARD CLOCKS ON A MONTHLY BASIS." Scientific Journal of Silesian University of Technology. Series Transport 114 (January 1, 2022): 193–209. http://dx.doi.org/10.20858/sjsutst.2022.114.16.
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This paper presents the stability of the GPS and GLONASS system clocks’ stability. It describes the construction of these two systems and calculated four different Allan variances (AVAR), based on the MGEX (the Multi-GNSS Experiment) clock products. Four used variances allowed making a better analysis of each GNSS system clock. The results are shown at different averaging times from 5 s as successive multiples to 655,360 s in a monthly period. The stability of GPS and GLONASS clocks is included in the range of 10-12~10-14 s. The results showed that GLONASS clocks are stable (10-12~10-14 s) and are affected with white frequency noise (WFM). The GPS clock stability models have more fluctuations for τ > 40,960 s and the mean stability is concluded between 10-12~10-13 s. Mean frequency accuracy for GPS clocks is related with WFM and Random Walk Frequency (RWF). The differences in clock stability are caused by several factors – block type, type of clock and the time of a satellite in orbit. These factors have an influence on stability results.
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Bravo, Tupac, Dennis Rätzel, and Ivette Fuentes. "Gravitational time dilation in extended quantum systems: The case of light clocks in Schwarzschild spacetime." AVS Quantum Science 5, no. 1 (March 2023): 014401. http://dx.doi.org/10.1116/5.0123228.
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The precision of optical atomic clocks is approaching a regime where they resolve gravitational time dilation on smaller scales than their own extensions. Hence, an accurate description of quantum clocks has to take their spatial extension into account. In this article, as a first step toward a fully relativistic description of extended quantum clocks, we investigate a quantized version of Einstein's light clock fixed at a constant distance from a large massive object like the Earth. The model consists of a quantum light field in a one-dimensional cavity in Schwarzschild spacetime, where the distance between the mirrors is fixed by a rigid rod. By comparing a vertical and a horizontal clock, we propose an operational way to define the clock time when the clock resolves gravitational time dilation on scales smaller than its extension. In particular, we show that the time measured by the vertical light clock is equivalent to the proper time defined at its center. We also derive fundamental bounds on the precision of these clocks for measurements of proper time and the Schwarzschild radius.
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Amaral, Ian P. G., and Ian A. Johnston. "Circadian expression of clock and putative clock-controlled genes in skeletal muscle of the zebrafish." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 302, no. 1 (January 2012): R193—R206. http://dx.doi.org/10.1152/ajpregu.00367.2011.
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To identify circadian patterns of gene expression in skeletal muscle, adult male zebrafish were acclimated for 2 wk to a 12:12-h light-dark photoperiod and then exposed to continuous darkness for 86 h with ad libitum feeding. The increase in gut food content associated with the subjective light period was much diminished by the third cycle, enabling feeding and circadian rhythms to be distinguished. Expression of zebrafish paralogs of mammalian transcriptional activators of the circadian mechanism ( bmal1, clock1, and rora) followed a rhythmic pattern with a ∼24-h periodicity. Peak expression of rora paralogs occurred at the beginning of the subjective light period [Zeitgeber time (ZT)07 and ZT02 for roraa and rorab], whereas the highest expression of bmal1 and clock paralogs occurred 12 h later (ZT13–15 and ZT16 for bmal and clock paralogs). Expression of the transcriptional repressors cry1a, per1a/1b, per2, per3, nr1d2a/2b, and nr1d1 also followed a circadian pattern with peak expression at ZT0–02. Expression of the two paralogs of cry2 occurred in phase with clock1a/1b. Duplicated genes had a high correlation of expression except for paralogs of clock1, nr1d2, and per1, with cry1b showing no circadian pattern. The highest expression difference was 9.2-fold for the activator bmal1b and 51.7-fold for the repressor per1a. Out of 32 candidate clock-controlled genes, only myf6, igfbp3, igfbp5b, and hsf2 showed circadian expression patterns. Igfbp3, igfbp5b, and myf6 were expressed in phase with clock1a/1b and had an average of twofold change in expression from peak to trough, whereas hsf2 transcripts were expressed in phase with cry1a and had a 7.2-fold-change in expression. The changes in expression of clock and clock-controlled genes observed during continuous darkness were also observed at similar ZTs in fish exposed to a normal photoperiod in a separate control experiment. The role of circadian clocks in regulating muscle maintenance and growth are discussed.
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Xu, Beizhen, Lei Wang, Wenju Fu, Ruizhi Chen, Tao Li, and Xinxin Zhang. "A Practical Adaptive Clock Offset Prediction Model for the Beidou-2 System." Remote Sensing 11, no. 16 (August 8, 2019): 1850. http://dx.doi.org/10.3390/rs11161850.
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The predicted navigation satellite clock offsets are crucial to support real-time global navigation satellite system (GNSS) precise positioning applications, especially for those applications difficult to access the real-time data stream, such as the low earth orbit (LEO) autonomous precise orbit determination. Currently, the clock prediction for the Chinese BeiDou system is still challenging to meet the precise positioning requirement. The onboard clocks of BeiDou satellites are provided by different manufacturers, and the clocks’ switch events are more frequent. Considering the satellite-specified and temporal variation of the BeiDou clocks characteristics, we intend to use an adaptive model for BeiDou clock prediction. During clock prediction, we identify different models for BeiDou clocks’ characteristics, and then address the optimal model with a cross-validation procedure. The model achieving the minimum variance in the cross-validation procedure is used for the final clock prediction. We compared the prediction results of our method with two well-recognized BeiDou ultra-rapid clock products, named GBU-P and ISU-P, respectively. The comparison results indicate that the adaptive model achieves about 1-ns precision for 3-h prediction, which corresponds to 47.3% and 32.1% precision improvement compared to the GBU-P and ISU-P products, respectively. The efficiency of the predicted clocks is further validated with the precise point positioning (PPP) data processing. The results indicate that the static PPP solution precision is improved by 21.6%–30.0% compared to the current predicted clock product. The precision improvement in kinematic PPP is even more significant, which reaches 46.7%–53.9% with respect to these GBU-P and ISU-P products. Therefore, the proposed adaptive model is a practical and an efficient way to improve the BeiDou clock prediction.
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Wang, Hu, Pengyuan Li, Jiexian Wang, Hongyang Ma, Yangfei Hou, and Yingying Ren. "Analysis of BDS-3 Real-Time Satellite Clock Offset Estimated in Global and Asia-Pacific and the Corresponding PPP Performances." Remote Sensing 14, no. 24 (December 7, 2022): 6206. http://dx.doi.org/10.3390/rs14246206.
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The quality of satellite clock offset affects the performances of positioning, navigation and timing services, and thus it is essential to the Global Navigation Satellite System (GNSS). This research focuses on the estimation of BeiDou Navigation Satellite System (BDS) real-time precise satellite clock offset by using GNSS stations located in the Global and Asia-Pacific region based on the mixed-difference model. The precision of the estimated BDS clock corrections is then analyzed with the classification of the orbit types, satellite generations, and atomic clock types. The results show that the precision of the BDS clock offset estimated in the Asia-Pacific for Geosynchronous Earth Orbit (GEO), Inclined Geosynchronous Satellite Orbit (IGSO) and Medium Earth Orbit (MEO) satellites are 0.204 ns, 0.077 ns and 0.085 ns, respectively, as compared to those of clock offsets estimated in globally distributed stations. The average precision of the BDS-3 satellites clock offset estimated in global region is 0.074 ns, which is much better than the 0.130 ns of BDS-2. Furthermore, analyzing the characteristics of the corresponding atomic clocks can explain the performance of the estimated satellite clock offset, and the stability and accuracy of various parameters of the Passive Hydrogen Maser (PHM) atomic clocks are better than those of Rubidium (Rb) atomic clocks. In the positioning domain, the real-time clocks estimated in the global/Asia-Pacific have been applied to BDS kinematic Precise Point Positioning (PPP) in different regions. The Root Mean Square (RMS) of positioning results in global real-time kinematic PPP is within 4 cm in the horizontal direction and about 6 cm in the vertical direction. Hence, the BDS real-time clock offset can supply the centimeter-level positioning demand around the world.
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Okamoto-Uchida, Yoshimi, Akari Nishimura, Junko Izawa, Atsuhiko Hattori, Nobuo Suzuki, and Jun Hirayama. "The Use of Chemical Compounds to Identify the Regulatory Mechanisms of Vertebrate Circadian Clocks." Current Drug Targets 21, no. 5 (April 20, 2020): 425–32. http://dx.doi.org/10.2174/1389450120666190926143120.
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Circadian clocks are intrinsic, time-tracking processes that confer a survival advantage on an organism. Under natural conditions, they follow approximately a 24-h day, modulated by environmental time cues, such as light, to maximize an organism’s physiological efficiency. The exact timing of this rhythm is established by cell-autonomous oscillators called cellular clocks, which are controlled by transcription–translation negative feedback loops. Studies of cell-based systems and wholeanimal models have utilized a pharmacological approach in which chemical compounds are used to identify molecular mechanisms capable of establishing and maintaining cellular clocks, such as posttranslational modifications of cellular clock regulators, chromatin remodeling of cellular clock target genes’ promoters, and stability control of cellular clock components. In addition, studies with chemical compounds have contributed to the characterization of light-signaling pathways and their impact on the cellular clock. Here, the use of chemical compounds to study the molecular, cellular, and behavioral aspects of the vertebrate circadian clock system is described.
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Gu, Shengfeng, Feiyu Mao, Xiaopeng Gong, Yidong Lou, Xueyong Xu, and Ye Zhou. "Evaluation of BDS-2 and BDS-3 Satellite Atomic Clock Products and Their Effects on Positioning." Remote Sensing 13, no. 24 (December 11, 2021): 5041. http://dx.doi.org/10.3390/rs13245041.
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The BeiDou Navigation Satellite System (BDS) has completed third phase construction and currently provides global services, with a mixed constellation of BDS-2 and BDS-3. The newly launched BDS-3 satellites are equipped with rubidium and passive hydrogen maser (PHM) atomic clocks. The performance of atomic clocks is one of the cores of satellite navigation system, which will affect the performance of positioning, navigation and timing (PNT). In this paper, we systematically analyze the characteristics of BDS-2 and BDS-3 atomic clocks, based on more than one year of precise satellite clock products and broadcast ephemeris. Firstly, the results of overlapping Allan variations demonstrate that BDS-3 Rb and PHM clocks improve better in stability than BDS-2 Rb clock and are comparable to GPS IIF Rb and Galileo PHM clocks. Accordingly, the STDs of BDS-3 broadcast satellite clock are better than GPS and BDS-2, which are at the same level with that of Galileo. Secondly, the inter-system bias (ISB) between BDS-2 and BDS-3 is analyzed by satellite clock datum comparison and precise point positioning (PPP). Surprisingly, the discrepancy between BDS-2 and BDS-3 satellite clock datum has a great difference between products that could reach up to about 10 ns for WHU satellite clock products and broadcast ephemeris. Moreover, the ISBs between BDS-2 and BDS-3 satellite clocks are quite stable over one-year periods. Thirdly, due to the improved stability of BDS-3 atomic clock, the 68% positioning accuracy is better than 0.65 m at 10 min for BDS-3 PPP, based on broadcast ephemeris. Besides, the non-negligible bias between BDS-2 and BDS-3 will greatly affect the BDS precise data processing. The accuracy of positioning is greatly improved when considering the ISB.
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Lartillot, Nicolas, Matthew J. Phillips, and Fredrik Ronquist. "A mixed relaxed clock model." Philosophical Transactions of the Royal Society B: Biological Sciences 371, no. 1699 (July 19, 2016): 20150132. http://dx.doi.org/10.1098/rstb.2015.0132.
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Over recent years, several alternative relaxed clock models have been proposed in the context of Bayesian dating. These models fall in two distinct categories: uncorrelated and autocorrelated across branches. The choice between these two classes of relaxed clocks is still an open question. More fundamentally, the true process of rate variation may have both long-term trends and short-term fluctuations, suggesting that more sophisticated clock models unfolding over multiple time scales should ultimately be developed. Here, a mixed relaxed clock model is introduced, which can be mechanistically interpreted as a rate variation process undergoing short-term fluctuations on the top of Brownian long-term trends. Statistically, this mixed clock represents an alternative solution to the problem of choosing between autocorrelated and uncorrelated relaxed clocks, by proposing instead to combine their respective merits. Fitting this model on a dataset of 105 placental mammals, using both node-dating and tip-dating approaches, suggests that the two pure clocks, Brownian and white noise, are rejected in favour of a mixed model with approximately equal contributions for its uncorrelated and autocorrelated components. The tip-dating analysis is particularly sensitive to the choice of the relaxed clock model. In this context, the classical pure Brownian relaxed clock appears to be overly rigid, leading to biases in divergence time estimation. By contrast, the use of a mixed clock leads to more recent and more reasonable estimates for the crown ages of placental orders and superorders. Altogether, the mixed clock introduced here represents a first step towards empirically more adequate models of the patterns of rate variation across phylogenetic trees. This article is part of the themed issue ‘Dating species divergences using rocks and clocks’.
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Xie, Wei, Guanwen Huang, Bobin Cui, Pingli Li, Yu Cao, Haohao Wang, Zi Chen, and Bo Shao. "Characteristics and Performance Evaluation of QZSS Onboard Satellite Clocks." Sensors 19, no. 23 (November 24, 2019): 5147. http://dx.doi.org/10.3390/s19235147.
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In the Global Navigation Satellite System (GNSS) community, the Quasi-Zenith Satellite System (QZSS) is an augmentation system for users in the Asia-Pacific region. However, the characteristics and performance of four QZSS satellite clocks in a long-term scale are unknown at present. However, it is crucial to the positioning, navigation and timing (PNT) services of users, especially in Asia-Pacific region. In this study, the characteristics and performance variation of four QZSS satellite clocks, which including the phase, frequency, frequency drift, fitting residuals, frequency accuracy, periodic terms, frequency stability and short-term clock prediction, are revealed in detail for the first time based on the precise satellite clock offset products of nearly 1000 days. The important contributions are as follows: (1) It is detected that the times of phase and frequency jump are 2.25 and 1.5 for every QZSS satellite clock in one year. The magnitude of the frequency drift is about 10−18. The periodic oscillation of frequency drift of J01 and J02 satellite clocks is found. The clock offset model precision of QZSS is 0.33 ns. (2) The two main periods of QZSS satellite clock are 24 and 12 hours, which is the influence of the satellite orbit; (3) The frequency stability of 100, 1000 and 10,000 s are 1.98 × 10−13, 6.59 × 10−14 and 5.39 × 10−14 for QZSS satellite clock, respectively. The visible “bump” is found at about 400 s for J02 and J03 satellite clocks. The short-term clock prediction accuracy of is 0.12 ns. This study provides a reference for the state monitoring and performance variation of the QZSS satellite clock.
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Ye, Aliaosha, Guoquan Zheng, Enguo Zhu, Shuai Hou, Qian Cheng, and Zhe Liu. "Research on Clock Acquisition and Correction Technology of Measurement Equipment." Journal of Physics: Conference Series 2360, no. 1 (November 1, 2022): 012005. http://dx.doi.org/10.1088/1742-6596/2360/1/012005.
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Comprehensive energy metering and electricity market-oriented transactions under the dual carbon goals put forward higher requirements on the accuracy of metering equipment clocks. The existing inventory of metering equipment has complex specifications, complex communication methods, and difficult management. Because of different production years, the old version of the metering equipment has poor clock retention capability and insufficient automatic time synchronization function. In order to solve the problems of clock accuracy and time management of metering equipment, a wide-area hierarchical collection and synchronization technology of clocks is designed, a technical framework for clock collection and synchronization of equipment at all levels of the system is constructed, and the normal state of system equipment is strengthened and maintained through the method of serial broadcast time calibration. Clock, designed a time-calibration protocol for object-oriented metering equipment. This technology has been applied to the clock management of on-site metering equipment, and the qualified rate of metering equipment clocks has reached more than 95%, which has improved the automation level of metering equipment clock management, the efficiency of time schooling, and the timely rate of time schooling.
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Patil, Archana. "Design and Simulation of Clock Divider using VHDL." INTERANTIONAL JOURNAL OF SCIENTIFIC RESEARCH IN ENGINEERING AND MANAGEMENT 08, no. 05 (May 12, 2024): 1–5. http://dx.doi.org/10.55041/ijsrem33837.
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This paper presents the diesign and simulation of clock divider circuit using VHDL(VHSIC Hardware Description Language) on an FPGA(Field Programmable Gate Array). The clock divider circuit is a fundamental component in digital system for generating lower frequency clocks from a higher frequency reference clock. The paper starts up with simple divider where the clock is divided by even numbers, odd numbers and then later expands it into non- integer dividers. Keywords:- clock divider, D flipflop, FPGA
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Liu, Shuai, Yunbin Yuan, Xiaosong Guo, Kezhi Wang, and Gongwei Xiao. "Cube: An Open-Source Software for Clock Offset Estimation and Precise Point Positioning with Ambiguity Resolution." Remote Sensing 16, no. 15 (July 26, 2024): 2739. http://dx.doi.org/10.3390/rs16152739.
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Precise point positioning (PPP) is a prevalent, high-precision spatial absolution positioning method, and its performance can be enhanced by ambiguity resolution (AR). To fulfill the growing need for high-precision positioning, we developed an open-source GNSS data processing package based on the decoupled clock model called Cube, which integrates decoupled clock offset estimation and precise point positioning with ambiguity resolution (PPP-AR). Cube is a secondary development based on RTKLIB. Besides the decoupled clock model, Cube can also estimate legacy clocks for the International GNSS Service (IGS), as well as clocks with satellite code bias extraction, and perform PPP-AR using the integer-recovered clock model. In this work, we designed satellite clock estimation and PPP-AR experiments with one week of GPS data to validate Cube’s performance. Results show that the software can produce high-precision satellite clock products and positioning results that are adequate for daily scientific study. With Cube, researchers do not need to rely on public PPP-AR products, and they can estimate decoupled clock products and implement PPP-AR anytime.
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Jiang, Gu, Li, Ge, and Schuh. "A Decentralized Processing Schema for Efficient and Robust Real-time Multi-GNSS Satellite Clock Estimation." Remote Sensing 11, no. 21 (November 5, 2019): 2595. http://dx.doi.org/10.3390/rs11212595.
Full textAbstract:
Real-time multi-GNSS precise point positioning (PPP) requires the support of high-rate satellite clock corrections. Due to the large number of ambiguity parameters, it is difficult to update clocks at high frequency in real-time for a large reference network. With the increasing number of satellites of multi-GNSS constellations and the number of stations, real-time high-rate clock estimation becomes a big challenge. In this contribution, we propose a decentralized clock estimation (DECE) strategy, in which both undifferenced (UD) and epoch-differenced (ED) mode are implemented but run separately in different computers, and their output clocks are combined in another process to generate a unique product. While redundant UD and/or ED processing lines can be run in offsite computers to improve the robustness, processing lines for different networks can also be included to improve the clock quality. The new strategy is realized based on the Position and Navigation Data Analyst (PANDA) software package and is experimentally validated with about 110 real-time stations for clock estimation by comparison of the estimated clocks and the PPP performance applying estimated clocks. The results of the real-time PPP experiment using 12 global stations show that with the greatly improved computational efficiency, 3.14 cm in horizontal and 5.51 cm in vertical can be achieved using the estimated DECE clock.
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Liu, Shuai, and Yunbin Yuan. "A Method to Accelerate the Convergence of Satellite Clock Offset Estimation Considering the Time-Varying Code Biases." Remote Sensing 13, no. 14 (July 9, 2021): 2714. http://dx.doi.org/10.3390/rs13142714.
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Continuous and stable precision satellite clock offsets are an important guarantee for real-time precise point positioning (PPP). However, in real-time PPP, the estimation of a satellite clock is often interrupted for various reasons such as network fluctuations, which leads to a long time for clocks to converge again. Typically, code biases are assumed to stay constant over time in clock estimation according to the current literature. In this contribution, it is shown that this assumption reduces the convergence speed of estimation, and the satellite clocks are still unstable for several hours after convergence. For this reason, we study the influence of different code bias extraction schemes, that is, taking code biases as constants, extracting satellite code biases (SCBs), extracting receiver code biases (RCBs) and simultaneously extracting SCBs and RCBs, on satellite clock estimation. Results show that, the time-varying SCBs are the main factors leading to the instability of satellite clocks, and considering SCBs in the estimation can significantly accelerate the filter convergence and improve the stability of clocks. Then, the products generated by introducing SCBs in the clock estimation based on undifferenced observations are applied to PPP experiments. Compared with the original undifferenced model, clocks estimated using the new method can significantly accelerate the convergence speed of PPP and improve the positioning accuracy, which illustrates that our estimated clocks are effective and superior.
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Xiao, Yangbo, Ye Yuan, Mariana Jimenez, Neeraj Soni, and Swathi Yadlapalli. "Clock proteins regulate spatiotemporal organization of clock genes to control circadian rhythms." Proceedings of the National Academy of Sciences 118, no. 28 (July 7, 2021): e2019756118. http://dx.doi.org/10.1073/pnas.2019756118.
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Circadian clocks regulate ∼24-h oscillations in gene expression, behavior, and physiology. While the genetic and molecular mechanisms of circadian rhythms are well characterized, what remains poorly understood are the intracellular dynamics of circadian clock components and how they affect circadian rhythms. Here, we elucidate how spatiotemporal organization and dynamics of core clock proteins and genes affect circadian rhythms in Drosophila clock neurons. Using high-resolution imaging and DNA-fluorescence in situ hybridization techniques, we demonstrate that Drosophila clock proteins (PERIOD and CLOCK) are organized into a few discrete foci at the nuclear envelope during the circadian repression phase and play an important role in the subnuclear localization of core clock genes to control circadian rhythms. Specifically, we show that core clock genes, period and timeless, are positioned close to the nuclear periphery by the PERIOD protein specifically during the repression phase, suggesting that subnuclear localization of core clock genes might play a key role in their rhythmic gene expression. Finally, we show that loss of Lamin B receptor, a nuclear envelope protein, leads to disruption of PER foci and per gene peripheral localization and results in circadian rhythm defects. These results demonstrate that clock proteins play a hitherto unexpected role in the subnuclear reorganization of core clock genes to control circadian rhythms, revealing how clocks function at the subcellular level. Our results further suggest that clock protein foci might regulate dynamic clustering and spatial reorganization of clock-regulated genes over the repression phase to control circadian rhythms in behavior and physiology.
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Singh, Pushpendra, Komal Saxena, Anup Singhania, Pathik Sahoo, Subrata Ghosh, Rutuja Chhajed, Kanad Ray, Daisuke Fujita, and Anirban Bandyopadhyay. "A Self-Operating Time Crystal Model of the Human Brain: Can We Replace Entire Brain Hardware with a 3D Fractal Architecture of Clocks Alone?" Information 11, no. 5 (April 27, 2020): 238. http://dx.doi.org/10.3390/info11050238.
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Time crystal was conceived in the 1970s as an autonomous engine made of only clocks to explain the life-like features of a virus. Later, time crystal was extended to living cells like neurons. The brain controls most biological clocks that regenerate the living cells continuously. Most cognitive tasks and learning in the brain run by periodic clock-like oscillations. Can we integrate all cognitive tasks in terms of running clocks of the hardware? Since the existing concept of time crystal has only one clock with a singularity point, we generalize the basic idea of time crystal so that we could bond many clocks in a 3D architecture. Harvesting inside phase singularity is the key. Since clocks reset continuously in the brain–body system, during reset, other clocks take over. So, we insert clock architecture inside singularity resembling brain components bottom-up and top-down. Instead of one clock, the time crystal turns to a composite, so it is poly-time crystal. We used century-old research on brain rhythms to compile the first hardware-free pure clock reconstruction of the human brain. Similar to the global effort on connectome, a spatial reconstruction of the brain, we advocate a global effort for more intricate mapping of all brain clocks, to fill missing links with respect to the brain’s temporal map. Once made, reverse engineering the brain would remain a mere engineering challenge.
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Bacelar Valente, Mario. "What do light clocks say to us regarding the so-called clock hypothesis?" THEORIA. An International Journal for Theory, History and Foundations of Science 33, no. 3 (November 6, 2018): 435. http://dx.doi.org/10.1387/theoria.18143.
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The clock hypothesis is taken to be an assumption independent of special relativity necessary to describe accelerated clocks. This enables to equate the time read off by a clock to the proper time. Here, it is considered a physical system–the light clock–proposed by Marzke and Wheeler. Recently, Fletcher proved a theorem that shows that a sufficiently small light clock has a time reading that approximates to an arbitrary degree the proper time. The clock hypothesis is not necessary to arrive at this result. Here, one explores the consequences of this regarding the status of the clock hypothesis.
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Wang, Kan, Ahmed El-Mowafy, and Xuhai Yang. "LEO Satellite Clock Modeling and Its Benefits for LEO Kinematic POD." Remote Sensing 15, no. 12 (June 16, 2023): 3149. http://dx.doi.org/10.3390/rs15123149.
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High-accuracy Low Earth Orbit (LEO) satellite clock and orbital products are preconditions to realize LEO augmentation for high-accuracy GNSS-based positioning on the ground. There is a high correlation between the orbit and clock parameters in the kinematic Precise Orbit Determination (POD) process. While future LEO satellites are planned to be equipped with better clocks, the benefits of modeling high-stability LEO satellite clocks are not yet thoroughly investigated, particularly when mid- to long-term systematic effects induced by the complex LEO relativistic effects and the external environment remain in the clocks. Through clock modeling, this study attempts to reduce not only the short-term noise of radial kinematic orbits, but also mis-modeled effects caused by, e.g., real-time GNSS orbital and clock errors. To explore the benefits of clock modeling, the clocks need to be first detrended by the mid- to long-term systematic effects. While over-detrending limits the orbital improvements, weak detrending would also hamper strong clock modeling and easily lead to performance degradations. A balance between the strengths of the detrending and the model thus needs to be investigated for different clock types. In this study, the Piece-Wise Linear (PWL) model of different time lengths and a 2.5-state filter with different strengths (h values) are tested using real data from GRACE FO-1 with an Ultra-Stable Oscillator (USO) on board. Using the CNES real-time GPS products, it was found that when detrending the clocks with a smoothing window of 300 to 500 s, one could generally expect an improvement larger than 10% in the estimation of radial orbits when applying a PWL model with a length from 300 to 1200 s. Improvements of this size can also be expected when using the 2.5-state model with h−1 (for Flicker Frequency Noise) from 10−28 to 10−30.
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Meng, Chaoyong, Chuanpei Xu, and Jiafeng Liao. "Research on Clock Synchronization of Data Acquisition Based on NoC." Applied Sciences 14, no. 11 (June 3, 2024): 4838. http://dx.doi.org/10.3390/app14114838.
Full textAbstract:
Data acquisition based on network-on-chip (NoC) technology is a high-sampling-rate data acquisition scheme using low-sampling-rate analog–digital conversion (ADC) chips. It has the characteristics of multi-task parallel communication, being global asynchronous, local synchronous clock distribution, high throughput, low transmission latency, and strong scalability. High-speed data acquisition is realized through the combination of an on-chip network and time-interleaved data acquisition technology. In the time-interleaved sampling technique, the precision of clock synchronization directly affects the precision of sampling. Based on the proposed NOC data acquisition scheme, an improved White Rabbit clock synchronization protocol is applied to high-speed data acquisition to achieve high-precision synchronization of multi-channel time-interleaved sampling clocks. Firstly, the offset of the master clock and slave clock is determined by the PTP protocol, and the offset is corrected to achieve rough synchronization between the master clock and slave clock. Secondly, a digital dual-mixer time difference (DDMTD) is used to measure the phases of the master and slave clocks. After that, the phase of the slave clock is corrected through the dynamic phase-shift function of the clock’s phase-locked loop (PLL). Finally, according to the simulation results in Modelsim, the average absolute error of a TI-ADC sampling clock can be less than 20 ps.
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Razzoli, Maria, Carley Karsten, J. Marina Yoder, Alessandro Bartolomucci, and William C. Engeland. "Chronic subordination stress phase advances adrenal and anterior pituitary clock gene rhythms." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 307, no. 2 (July 15, 2014): R198—R205. http://dx.doi.org/10.1152/ajpregu.00101.2014.
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Circadian rhythms in glucocorticoids are the product of interactions between the hypothalamic-pituitary-adrenal (HPA) axis and the mammalian clock gene system. The adrenal clock can generate the glucocorticoid rhythm that in turn synchronizes other peripheral clocks to maintain homeostasis. Stress acutely activates and chronically upregulates the HPA axis, suggesting that the adrenal clock could be modulated by stress. However, there is no direct evidence that stress affects the adrenal clock rhythm. We tested the hypothesis that a model of chronic subordination stress (CSS) that has a major impact on HPA axis regulation, metabolism, and emotional behavior alters adrenal and pituitary clock gene rhythms. Clock gene rhythms were assessed using mPER2::Luciferase (PER2Luc) knockin mice in which in vitro bioluminescence rhythms reflect the Per2 clock gene expression. PER2Luc mice that experienced CSS for 2 wk showed positive energy balance reflected by increased body weight and food intake. Additionally, CSS phase advanced the adrenal (∼2 h) and the pituitary (∼1 h) PER2Luc rhythm compared with control mice. The activity rhythm was not affected. The adrenal clock phase shift was associated with increased feed conversion efficiency, suggesting that the metabolic phenotype in CSS mice may be related to altered adrenal clock rhythmicity. Interestingly, a single subordination experience followed by 8 h sensory housing also phase advanced the adrenal, but not the pituitary, PER2Luc rhythm. Overall, these data demonstrate a stress-induced phase shift in a peripheral clock gene rhythm and differential stress sensitivity of two peripheral clocks within the HPA axis, suggesting a link between clock desynchrony and individual vulnerability to stress.
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Min, Mildred, Caitlin Egli, and Raja K. Sivamani. "The Gut and Skin Microbiome and Its Association with Aging Clocks." International Journal of Molecular Sciences 25, no. 13 (July 8, 2024): 7471. http://dx.doi.org/10.3390/ijms25137471.
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Aging clocks are predictive models of biological age derived from age-related changes, such as epigenetic changes, blood biomarkers, and, more recently, the microbiome. Gut and skin microbiota regulate more than barrier and immune function. Recent studies have shown that human microbiomes may predict aging. In this narrative review, we aim to discuss how the gut and skin microbiomes influence aging clocks as well as clarify the distinction between chronological and biological age. A literature search was performed on PubMed/MEDLINE databases with the following keywords: “skin microbiome” OR “gut microbiome” AND “aging clock” OR “epigenetic”. Gut and skin microbiomes may be utilized to create aging clocks based on taxonomy, biodiversity, and functionality. The top contributing microbiota or metabolic pathways in these aging clocks may influence aging clock predictions and biological age. Furthermore, gut and skin microbiota may directly and indirectly influence aging clocks through the regulation of clock genes and the production of metabolites that serve as substrates or enzymatic regulators. Microbiome-based aging clock models may have therapeutic potential. However, more research is needed to advance our understanding of the role of microbiota in aging clocks.
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Korvorst, Marjolein, Ardi Roelofs, and Willem J. M. Levelt. "Telling Time from Analog and Digital Clocks." Experimental Psychology 54, no. 3 (January 2007): 187–91. http://dx.doi.org/10.1027/1618-3169.54.3.187.
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Abstract. Does the naming of clocks always require conceptual preparation? To examine this question, speakers were presented with analog and digital clocks that had to be named in Dutch using either a relative (e.g., “quarter to four”) or an absolute (e.g., “three forty-five”) clock time expression format. Naming latencies showed evidence of conceptual preparation when speakers produced relative time expressions to analog and digital clocks, but not when they used absolute time expressions. These findings indicate that conceptual mediation is not always mandatory for telling time, but instead depends on clock time expression format, supporting a multiple-route account of Dutch clock time naming.
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Helfrich-Förster, Charlotte, Michael N. Nitabach, and Todd C. Holmes. "Insect circadian clock outputs." Essays in Biochemistry 49 (June 30, 2011): 87–101. http://dx.doi.org/10.1042/bse0490087.
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Insects display an impressive variety of daily rhythms, which are most evident in their behaviour. Circadian timekeeping systems that generate these daily rhythms of physiology and behaviour all involve three interacting elements: the timekeeper itself (i.e. the clock), inputs to the clock through which it entrains and otherwise responds to environmental cues such as light and temperature, and outputs from the clock through which it imposes daily rhythms on various physiological and behavioural parameters. In insects, as in other animals, cellular clocks are embodied in clock neurons capable of sustained autonomous circadian rhythmicity, and those clock neurons are organized into clock circuits. Drosophila flies spend their entire lives in small areas near the ground, and use their circadian brain clock to regulate daily rhythms of rest and activity, so as to organize their behaviour appropriately to the daily rhythms of their local environment. Migratory locusts and butterflies, on the other hand, spend substantial portions of their lives high up in the air migrating long distances (sometimes thousands of miles) and use their circadian brain clocks to provide time-compensation to their sun-compass navigational systems. Interestingly, however, there appear to be substantial similarities in the cellular and network mechanisms that underlie circadian outputs in all insects.
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Fu, Minnie, and Xiaoyong Yang. "The sweet tooth of the circadian clock." Biochemical Society Transactions 45, no. 4 (July 3, 2017): 871–84. http://dx.doi.org/10.1042/bst20160183.
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The endogenous circadian clock is a key regulator of daily metabolic processes. On the other hand, circadian clocks in a broad range of tissues can be tuned by extrinsic and intrinsic metabolic cues. The bidirectional interaction between circadian clocks and metabolism involves both transcriptional and post-translational mechanisms. Nuclear receptors exemplify the transcriptional programs that couple molecular clocks to metabolism. The post-translational modifications of the core clock machinery are known to play a key role in metabolic entrainment of circadian clocks. O-linked N-acetylglucosamine modification (O-GlcNAcylation) of intracellular proteins is a key mediator of metabolic response to nutrient availability. This review highlights our current understanding of the role of protein O-GlcNAcylation in mediating metabolic input and output of the circadian clock.
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Lee, Michael S. Y. "Multiple morphological clocks and total-evidence tip-dating in mammals." Biology Letters 12, no. 7 (July 2016): 20160033. http://dx.doi.org/10.1098/rsbl.2016.0033.
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Morphological integration predicts that correlated characters will coevolve; thus, each distinct suite of correlated characters might be expected to evolve according to a separate clock or ‘pacemaker’. Characters in a large morphological dataset for mammals were found to be evolving according to seven separate clocks, each distinct from the molecular clock. Total-evidence tip-dating using these multiple clocks inflated divergence time estimates, but potentially improved topological inference. In particular, single-clock analyses placed several meridiungulates and condylarths in a heterodox position as stem placentals, but multi-clock analyses retrieved a more plausible and orthodox position within crown placentals. Several shortcomings (including uneven character sampling) currently impact upon the accuracy of total-evidence dating, but this study suggests that when sufficiently large and appropriately constructed phenotypic datasets become more commonplace, multi-clock approaches are feasible and can affect both divergence dates and phylogenetic relationships.
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Halmos, Tamás, and Ilona Suba. "Physiological and pathophysiological role of the circadian clock system." Orvosi Hetilap 153, no. 35 (September 2012): 1370–79. http://dx.doi.org/10.1556/oh.2012.29436.
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It has been well known for ages that in living organisms the rhythmicity of biological processes is linked to the ~ 24-hour light–dark cycle. However, the exact function of the circadian clock system has been explored only in the past decades. It came to light that the photosensitive primary “master clock” is situated in the suprachiasmatic photosensitive nuclei of the special hypothalamic region, and that it is working according to ~24-hour changes of light and darkness. The master clock sends its messages to the peripheral “slave clocks”. In many organs, like pancreatic β-cells, the slave clocks have autonomic functions as well. Two essential components of the clock system are proteins encoded by the CLOCK and BMAL1 genes. CLOCK genes are in interaction with endonuclear receptors such as peroxisoma-proliferator activated receptors and Rev-erb-α, as well as with the hypothalamic-pituitary-adrenal axis, regulating the adaptation to stressors, energy supply, metabolic processes and cardiovascular system. Melatonin, the product of corpus pineale has a significant role in the functions of the clock system. The detailed discovery of the clock system has changed our previous knowledge about the development of many diseases. The most explored fields are hypertension, cardiovascular diseases, metabolic processes, mental disorders, cancers, sleep apnoe and joint disorders. CLOCK genes influence ageing as well. The recognition of the periodicity of biological processes makes the optimal dosing of certain drugs feasible. The more detailed discovery of the interaction of the clock system might further improve treatment and prevention of many disorders. Orv. Hetil., 2012, 153, 1370–1379.
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Sfarti, Adrian. "Relativistic light clocks: Arbitrary orientation in uniform motion and hyperbolic motion analysis." Journal of AppliedMath 2, no. 2 (June 12, 2024): 405. http://dx.doi.org/10.59400/jam.v2i2.405.
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In this paper, we address the general case of a light clock in uniform translational motion parallel to itself and perpendicular to its uniform velocity v, as well as the case of the light clock in relativistic hyperbolic motion. Neither case has been previously addressed in the specialized literature, which typically restricts itself to canonical orientations where the light clock moves parallel to either the vertical or horizontal axis with uniform velocity, without acceleration. Therefore, it becomes interesting to study the more general case where the clock has an arbitrary orientation and/or is accelerated. Our paper is divided into two main sections. The first section deals with the light clock moving with constant velocity, oriented at an arbitrary angle with respect to the x-axis. We prove that the moving clock exhibits a standard time dilation, identical to that of a light clock moving in a canonical orientation. The second section deals with the light clock moving with constant acceleration, i.e., in hyperbolic motion. For the light clock in hyperbolic motion, we derive the period as measured from the perspective of an inertial frame and draw parallels with the case of uniform motion, outlining a term that is similar (but not identical) to the γ factor of uniform motion. We also point out that this factor depends not only on acceleration but also on the height of the light clock. This dependency on the dimension of the light clock distinguishes the accelerated case from the case of uniform motion. The first three sections deal with the theoretical aspects of light (optical) clocks, while the fourth section addresses the experimental implementations of optical clocks.
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Costello, Hannah M., and Michelle L. Gumz. "Circadian Rhythm, Clock Genes, and Hypertension: Recent Advances in Hypertension." Hypertension 78, no. 5 (November 2021): 1185–96. http://dx.doi.org/10.1161/hypertensionaha.121.14519.
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Accumulating evidence suggests that the molecular circadian clock is crucial in blood pressure (BP) control. Circadian rhythms are controlled by the central clock, which resides in the suprachiasmatic nucleus of the hypothalamus and peripheral clocks throughout the body. Both light and food cues entrain these clocks but whether these cues are important for the circadian rhythm of BP is a growing area of interest. The peripheral clocks in the smooth muscle, perivascular adipose tissue, liver, adrenal gland, and kidney have been recently implicated in the regulation of BP rhythm. Dysregulation of the circadian rhythm of BP is associated with adverse cardiorenal outcomes and increased risk of cardiovascular mortality. In this review, we summarize the most recent advances in peripheral clocks as BP regulators, highlight the adverse outcomes of disrupted circadian BP rhythm in hypertension, and provide insight into potential future work in areas exploring the circadian clock in BP control and chronotherapy. A better understanding of peripheral clock function in regulating the circadian rhythm of BP will help pave the way for targeted therapeutics in the treatment of circadian BP dysregulation and hypertension.
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Li, Meina, Lijun Cao, Musoki Mwimba, Yan Zhou, Ling Li, Mian Zhou, Patrick S. Schnable, Jamie A. O’Rourke, Xinnian Dong, and Wei Wang. "Comprehensive mapping of abiotic stress inputs into the soybean circadian clock." Proceedings of the National Academy of Sciences 116, no. 47 (November 1, 2019): 23840–49. http://dx.doi.org/10.1073/pnas.1708508116.
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The plant circadian clock evolved to increase fitness by synchronizing physiological processes with environmental oscillations. Crop fitness was artificially selected through domestication and breeding, and the circadian clock was identified by both natural and artificial selections as a key to improved fitness. Despite progress in Arabidopsis, our understanding of the crop circadian clock is still limited, impeding its rational improvement for enhanced fitness. To unveil the interactions between the crop circadian clock and various environmental cues, we comprehensively mapped abiotic stress inputs to the soybean circadian clock using a 2-module discovery pipeline. Using the “molecular timetable” method, we computationally surveyed publicly available abiotic stress-related soybean transcriptomes to identify stresses that have strong impacts on the global rhythm. These findings were then experimentally confirmed using a multiplexed RNA sequencing technology. Specific clock components modulated by each stress were further identified. This comprehensive mapping uncovered inputs to the plant circadian clock such as alkaline stress. Moreover, short-term iron deficiency targeted different clock components in soybean and Arabidopsis and thus had opposite effects on the clocks of these 2 species. Comparing soybean varieties with different iron uptake efficiencies suggests that phase modulation might be a mechanism to alleviate iron deficiency symptoms in soybean. These unique responses in soybean demonstrate the need to directly study crop circadian clocks. Our discovery pipeline may serve as a broadly applicable tool to facilitate these explorations.
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Qin, Cheng-Gang, Tong Liu, Jin-Zhuang Dong, Xiao-Yi Dai, Yu-Jie Tan, and Cheng-Gang Shao. "Tidal Effects and Clock Comparison Experiments." Universe 9, no. 3 (March 4, 2023): 133. http://dx.doi.org/10.3390/universe9030133.
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Einstein’s general relativity theory provides a successful understanding of the flow of time in the gravitational field. From Einstein’s equivalence principle, the influence of the Sun and Moon masses on clocks is given in the form of tidal potentials. Two clocks fixed on the surface of the Earth, compared to each other, can measure the tidal effects of the Sun and Moon. The measurement of tidal effects can provide a test for general relativity. Based on the standard general relativity method, we rigorously derive the formulas for clock comparison in the Barycentric Celestial Reference System and Geocentric Celestial Reference System, and demonstrate the tidal effects on clock comparison experiments. The unprecedented performance of atomic clocks makes it possible to measure the tidal effects on clock comparisons. We propose to test tidal effects with the laboratory clock comparisons and some international missions, and give the corresponding estimations. By comparing the state-of-the-art clocks over distances of 1000 km, the laboratory may test tidal effects with a level of 1%. Future space missions, such as the China space station and FOCOS mission, can also be used to test tidal effects, and the best accuracy may reach 0.3%.
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Clark, Amelia M., and Brian J. Altman. "Circadian control of macrophages in the tumor microenvironment." Journal of Immunology 208, no. 1_Supplement (May 1, 2022): 165.06. http://dx.doi.org/10.4049/jimmunol.208.supp.165.06.
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Abstract Introduction All leukocytes tested to date have functional circadian clocks, and nearly every arm of the immune response is subject to circadian regulation. Circadian clocks instruct the time-of-day-dependent, rhythmic expression of genes in a tissue- and cell-specific manner. In macrophages (mΦs), the circadian clock regulates several factors that are critical to executing effective immune responses. Tumor-associated mΦs are major contributors to immune suppression in the tumor microenvironment (TME). Evidence suggests that metabolically stressful factors in the TME such as acidic pH and nutrient limitation promote mΦ-mediated immune suppression, and recent data point to dysregulation of the circadian clock downstream of metabolic stress. Methods We study the effect of TME-associated metabolic stress on the circadian clock of mΦs in vitro by culturing bone marrow-derived mΦs in conditions mimicking acidic pH and nutrient limitations that have been observed in the TME. To study the impact of mΦ-intrinsic circadian rhythms on tumorigenesis in vivo, we use mice genetically engineered to have a myeloid cell-specific disruption of the circadian clock via deletion of the key clock protein BMAL1. Results Oscillation of core clock proteins is altered in mΦs subjected to TME-associated metabolic stress. Additionally, we observe increased tumor growth in mice co-injected with mΦs whose circadian clocks were disrupted compared to mice co-injected with mΦs whose circadian clocks were functional. Conclusion Our data suggests that stressful conditions associated with the TME can alter the mΦ circadian clock, and that a functional circadian clock in mΦs can suppress tumor growth in a syngeneic murine tumor model of pancreatic cancer. This research has been supported by the following fellowships and grants: 2021-Current: Wilmot Predoctoral Cancer Research Fellowship, Wilmot Cancer Institute, University of Rochester Medical Center, Rochester, NY 2020-2021: NIH T32 Training Grant in Cellular, Biochemical & Molecular Sciences, University of Rochester Medical Center, Rochester, NY
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James, Allan B., José A. Monreal, Gillian A. Nimmo, Ciarán L. Kelly, Pawel Herzyk, Gareth I. Jenkins, and Hugh G. Nimmo. "The Circadian Clock inArabidopsisRoots Is a Simplified Slave Version of the Clock in Shoots." Science 322, no. 5909 (December 19, 2008): 1832–35. http://dx.doi.org/10.1126/science.1161403.
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The circadian oscillator in eukaryotes consists of several interlocking feedback loops through which the expression of clock genes is controlled. It is generally assumed that all plant cells contain essentially identical and cell-autonomous multiloop clocks. Here, we show that the circadian clock in the roots of matureArabidopsisplants differs markedly from that in the shoots and that the root clock is synchronized by a photosynthesis-related signal from the shoot. Two of the feedback loops of the plant circadian clock are disengaged in roots, because two key clock components, the transcription factors CCA1 and LHY, are able to inhibit gene expression in shoots but not in roots. Thus, the plant clock is organ-specific but not organ-autonomous.
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Lee, Kwangjun, Kwang-Seok Hong, Jonghoon Park, and Wonil Park. "Readjustment of circadian clocks by exercise intervention is a potential therapeutic target for sleep disorders: a narrative review." Physical Activity and Nutrition 28, no. 2 (June 30, 2024): 35–42. http://dx.doi.org/10.20463/pan.2024.0014.
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[Purpose] Circadian clocks are evolved endogenous biological systems that communicate with environmental cues to optimize physiological processes, such as the sleep-wake cycle, which is nearly related to quality of life. Sleep disorders can be treated using pharmacological strategies targeting melatonin, orexin, or core clock genes. Exercise has been widely explored as a behavioral treatment because it challenges homeostasis in the human body and affects the regulation of core clock genes. Exercise intervention at the appropriate time of the day can induce a phase shift in internal clocks. Although exercise is a strong external time cue for resetting the circadian clock, exercise therapy for sleep disorders remains poorly understood.[Methods] This review focused on exercise as a potential treatment for sleep disorders by tuning the internal circadian clock. We used scientific paper depositories, including Google Scholar, PubMed, and the Cochrane Library, to identify previous studies that investigated the effects of exercise on circadian clocks and sleep disorders.[Results] The exercise-induced adjustment of the circadian clock phase depended on exercise timing and individual chronotypes. Adjustment of circadian clocks through scheduled morning exercises can be appropriately prescribed for individuals with delayed sleep phase disorders. Individuals with advanced sleep phase disorders can synchronize their internal clocks with their living environment by performing evening exercises. Exercise-induced physiological responses are affected by age, sex, and current fitness conditions.[Conclusion] Personalized approaches are necessary when implementing exercise interventions for sleep disorders.
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Popkonstantinović, Branislav, Ratko Obradović, Miša Stojićević, Zorana Jeli, Ivana Cvetković, Ivana Vasiljević, and Zoran Milojević. "The Design and Simulation of an Astronomical Clock." Applied Sciences 11, no. 9 (April 28, 2021): 3989. http://dx.doi.org/10.3390/app11093989.
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This paper describes and explains the synthesis of an astronomical clock mechanism which displays the mean position of the Sun, the Moon, the lunar node and zodiac circle as well as the Moon phases and their motion during the year as seen from the Earth. The clock face represents the stereographic projection of the celestial equator, celestial tropics, zodiac circle (ecliptic) and horizon for the latitude of Belgrade from the north celestial pole to the equator plane. The observed motions of celestial objects are realized by a set of clock gear trains with properly calculated gear ratios. The method of continued fraction is applied in the computation of proper and practically applicable gear ratios of the clock gear trains. The fully operational 3D model of the astronomical clock is created and the motion study of its operation is accomplished by using the SolidWorks 2016 application. The simulation results are compared with the ephemeris data and the detected differences are used to evaluate the long-term accuracy of the astronomical clock operation. The presented methods of the clock mechanism synthesis can be useful for the design, maintenance and conservation of large-scale city astronomical clocks since these clocks represent a precious historical and cultural heritage of European civilization.