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Eberhard Gwinner - One of the best experts on this subject based on the ideXlab platform.
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Is food availability a circannual Zeitgeber in tropical birds? A field experiment on stonechats in tropical Africa.
Journal of biological rhythms, 2002Co-Authors: Alexander Scheuerlein, Eberhard GwinnerAbstract:Equatorial stonechats (Saxicola torquata axillaris) in Africa are seasonal breeders like their temperate-zone conspecifics (S.t. rubicola). Their annual cycle in gonadal size and function is controlled by an endogenous circannual rhythmicity that has been shown to run for up to 10 years in a constant equatorial photoperiod under laboratory conditions, with a period deviating from 12 months. In nature, however, this rhythm is synchronized with the actual year. Because photoperiod is essentially constant at the equator, it is likely that other environmental factors act as Zeitgebers. The authors test whether food availability affects reproductive cycles of free-living East African stonechats. The authors offered supplemental food to the birds 2 months before the regular onset of the breeding season. Supplementally fed males started to sing and display earlier than males of control pairs that did not receive extra food. Although the supplemented food advanced the onset of the breeding season in the pairs that were fed, the onset of the postnuptial molt following the breeding season was not correspondingly shifted. Furthermore, in the year following the experiment, all pairs initiated breeding at the same time. The authors conclude that food availability does not act as a Zeitgeber, but rather as a factor that modifies the timing of reproduction without affecting the underlying rhythmicity. The authors propose that this is adaptive under environmental conditions that are relatively constant within a given year but may vary considerably between years. The Zeitgeber synchronizing the endogenous rhythmicity remains to be identified.
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Exogenous melatonin reduces the resynchronization time after phase shifts of a nonphotic Zeitgeber in the house sparrow (Passer domesticus).
Journal of biological rhythms, 2000Co-Authors: Ute Abraham, Eberhard Gwinner, T. J. Van't HofAbstract:Continuous melatonin administration via silastic implants accelerates the resynchronization of the circadian locomotor activity rhythm in house sparrows (Passer domesticus) after exposure to phase shifts of a weak light-dark cycle. Constant melatonin might induce this effect either by increasing the sensitivity of the visual system to a light Zeitgeber or by reducing the degree of self-sustainment of the circadian pacemaker. To distinguish between these two possible mechanisms, two groups of house sparrows, one carrying melatonin implants and the other empty implants, were kept in constant dim light and subjected to advance and delay shifts of a 12-h feeding phase. The resynchronization times of their circadian feeding rhythm following the phase shifts were significantly shorter when the birds carried melatonin implants than when they carried empty implants. In a second experiment, melatonin-implanted and control birds were released into food ad libitum conditions 2 days after either a delay or an advance phase shift. The number of hours by which the activity rhythms had been shifted on the second day in food ad libitum conditions was assessed. Melatonin-implanted house sparrows had significantly larger phase shifts in their circadian feeding rhythm than control birds. This is in accordance with the first experiment since a larger phase shift at a given time reflects accelerated resynchronization. Additionally, the second experiment also excludes any possible masking effects of the nonphotic Zeitgeber. In conclusion, constant melatonin accelerates resynchronization even after phase shifts of a nonphotic Zeitgeber, indicating that constant high levels of melatonin can reduce the degree of self-sustainment of the circadian pacemaker independent of any effects on the photoreceptive system.
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Adjustment of house sparrow circadian rhythms to a simultaneously applied light and food Zeitgeber.
Physiology & behavior, 1997Co-Authors: Michaela Hau, Eberhard GwinnerAbstract:Periodic food availability has been shown to be an effective circadian Zeitgeber in many vertebrates. It is still unclear, however, i) whether light-active species like most birds can synchronize with food cycles in the presence of a strong light-dark (LD) cycle and ii) whether it is common among non-mammalian vertebrates to use a separate circadian oscillator to synchronize with food cycles as most mammals do. We investigated these questions experimentally by exposing house sparrows simultaneously to two Zeitgebers: light and food. The LD cycle was set at 1410 h; food was always available for 12 hour per day, but at different phases of the LD cycle. The effects of the two Zeitgebers were analyzed by observing two behavioral outputs of the birds' circadian system, the rhythms of locomotion and feeding. The data revealed that light acted as the dominant Zeitgeber in most conditions. Food cycles affected the phase of the behavioral rhythms of the birds only when the food was presented no later than 3 h after the onset of light. Apart from their synchronizing actions both light and food cycles also exerted direct (masking) effects on the behavioral rhythms of the birds. The results suggest that the circadian system of house sparrows can indeed adjust to two simultaneous environmental periodicities, i.e. light and food. We propose that light is the stronger Zeitgeber and plays a 'permissive' role in determining the phases at which synchronization with food cycles comes into effect. We did not find evidence that the house sparrows' behavioral rhythms are controlled by a food-entrainable circadian oscillator that is distinct from the light-entrainable system as is the case in most mammals. The differences in the patterns of food synchronization and organization of the circadian system that appear to exist between different species can be interpreted in two ways: i) species of different phylogenetic origin (e.g., mammals versus birds) evolved different circadian system or ii) regardless of phylogeny, species with different ecological requirements show a specialization in their circadian organization which is adjusted to the importance of Zeitgebers in nature.
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Food as a circadian Zeitgeber for house sparrows: The effect of different food access durations
Journal of biological rhythms, 1996Co-Authors: Michaela Hau, Eberhard GwinnerAbstract:House sparrows (Passer domesticus) can synchronize their circadian rhythms of locomotion and feeding with times of periodic food availability. In contrast to most mammals, which synchronize only a specific part of their circadian system with feeding cycles and thus express two distinct activity components, house sparrows synchronize their circadian activity rhythms as a whole with the food Zeitgeber. Previous results had indicated that feeding cycles act as comparatively weak Zeitgebers for house sparrows. In the present study, therefore, we investigate whether feeding schedules are weak Zeitgebers in general or whether their impact on the circadian system of the birds depends on the degree of food restriction. A detailed analysis of the synchronization pattern under the different experimental conditions should help to clarify whether house sparrows use a different mechanism for food-synchronization than mammals. House sparrows were kept in continuous dim light and exposed to different feeding schedules with daily food access durations ranging from 8 to 20 h. Many birds lost synchronization and exhibited free-running rhythms in locomotor and feeding activity when the daily food access duration was lengthened but became synchronized when the feeding duration was shortened. The interpretation that short food access durations represent stronger Zeitgebers than long food access durations was supported by the occurrence of large negative phase-angle differences during long daily feeding schedules, contrasting with small and sometimes positive phase-angle differences under short food access durations. There were no indications that house sparrows possess a specific food-entrainable circadian oscillator as mammals do. Rather, periodic food availability seems to be a Zeitgeber for the whole circadian system, which, hence, can be synchronized both by light and food. An explanation for such different mechanisms of food-synchronization is offered in the feeding ecology of these animals. Birds may evaluate the importance of a specific feeding schedule as a Zeitgeber either from temporal information on the duration of the daily food access time or from energetic considerations. The phase-angle differences associated with the different feeding schedules and the maintenance of daily activity times may ensure an appropriate temporal integration of behavior with specific conditions. Non-synchronized birds exhibited masking-induced feeding activity, which might represent an alternative means of adjusting to feeding cycles when synchronization cannot occur.
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continuous melatonin administration accelerates resynchronization following phase shifts of a light dark cycle
Physiology & Behavior, 1995Co-Authors: Michaela Hau, Eberhard GwinnerAbstract:Circadian rhythms of most passerine birds and some reptiles depend to a considerable extent on the presence of periodic melatonin, which is considered to be part of the central pacemaking system. Recent results on house sparrows have suggested that melatonin, apart from its periodic effects on the circadian system, may also exert effects derived from a constant action: Continuous melatonin applied through subcutaneously implanted silastic tubing enhanced the synchronization response to a low-amplitude light-dark Zeitgeber, indicating some kind of sensitization to Zeitgeber stimuli. In the present study we tested the prediction derived from these results, that speed of resynchronization after a phase shift of a light-cycle should be enhanced if melatonin is continuously administered. We found that, indeed, house sparrows required less time to resynchronize to an 8 h advance or delay phase shift of a low-amplitude light-dark cycle while carrying a silastic implant filled with melatonin than while carrying an empty implant. The effect is suggested to result from either one or a combination of the following mechanisms: (i) An increased circadian visual sensitivity, (ii) a diminished amplitude of the circadian oscillation, iii) an altered feedback of the locomotor activity to the oscillatory system.
Michaela Hau - One of the best experts on this subject based on the ideXlab platform.
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Adjustment of house sparrow circadian rhythms to a simultaneously applied light and food Zeitgeber.
Physiology & behavior, 1997Co-Authors: Michaela Hau, Eberhard GwinnerAbstract:Periodic food availability has been shown to be an effective circadian Zeitgeber in many vertebrates. It is still unclear, however, i) whether light-active species like most birds can synchronize with food cycles in the presence of a strong light-dark (LD) cycle and ii) whether it is common among non-mammalian vertebrates to use a separate circadian oscillator to synchronize with food cycles as most mammals do. We investigated these questions experimentally by exposing house sparrows simultaneously to two Zeitgebers: light and food. The LD cycle was set at 1410 h; food was always available for 12 hour per day, but at different phases of the LD cycle. The effects of the two Zeitgebers were analyzed by observing two behavioral outputs of the birds' circadian system, the rhythms of locomotion and feeding. The data revealed that light acted as the dominant Zeitgeber in most conditions. Food cycles affected the phase of the behavioral rhythms of the birds only when the food was presented no later than 3 h after the onset of light. Apart from their synchronizing actions both light and food cycles also exerted direct (masking) effects on the behavioral rhythms of the birds. The results suggest that the circadian system of house sparrows can indeed adjust to two simultaneous environmental periodicities, i.e. light and food. We propose that light is the stronger Zeitgeber and plays a 'permissive' role in determining the phases at which synchronization with food cycles comes into effect. We did not find evidence that the house sparrows' behavioral rhythms are controlled by a food-entrainable circadian oscillator that is distinct from the light-entrainable system as is the case in most mammals. The differences in the patterns of food synchronization and organization of the circadian system that appear to exist between different species can be interpreted in two ways: i) species of different phylogenetic origin (e.g., mammals versus birds) evolved different circadian system or ii) regardless of phylogeny, species with different ecological requirements show a specialization in their circadian organization which is adjusted to the importance of Zeitgebers in nature.
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Food as a circadian Zeitgeber for house sparrows: The effect of different food access durations
Journal of biological rhythms, 1996Co-Authors: Michaela Hau, Eberhard GwinnerAbstract:House sparrows (Passer domesticus) can synchronize their circadian rhythms of locomotion and feeding with times of periodic food availability. In contrast to most mammals, which synchronize only a specific part of their circadian system with feeding cycles and thus express two distinct activity components, house sparrows synchronize their circadian activity rhythms as a whole with the food Zeitgeber. Previous results had indicated that feeding cycles act as comparatively weak Zeitgebers for house sparrows. In the present study, therefore, we investigate whether feeding schedules are weak Zeitgebers in general or whether their impact on the circadian system of the birds depends on the degree of food restriction. A detailed analysis of the synchronization pattern under the different experimental conditions should help to clarify whether house sparrows use a different mechanism for food-synchronization than mammals. House sparrows were kept in continuous dim light and exposed to different feeding schedules with daily food access durations ranging from 8 to 20 h. Many birds lost synchronization and exhibited free-running rhythms in locomotor and feeding activity when the daily food access duration was lengthened but became synchronized when the feeding duration was shortened. The interpretation that short food access durations represent stronger Zeitgebers than long food access durations was supported by the occurrence of large negative phase-angle differences during long daily feeding schedules, contrasting with small and sometimes positive phase-angle differences under short food access durations. There were no indications that house sparrows possess a specific food-entrainable circadian oscillator as mammals do. Rather, periodic food availability seems to be a Zeitgeber for the whole circadian system, which, hence, can be synchronized both by light and food. An explanation for such different mechanisms of food-synchronization is offered in the feeding ecology of these animals. Birds may evaluate the importance of a specific feeding schedule as a Zeitgeber either from temporal information on the duration of the daily food access time or from energetic considerations. The phase-angle differences associated with the different feeding schedules and the maintenance of daily activity times may ensure an appropriate temporal integration of behavior with specific conditions. Non-synchronized birds exhibited masking-induced feeding activity, which might represent an alternative means of adjusting to feeding cycles when synchronization cannot occur.
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continuous melatonin administration accelerates resynchronization following phase shifts of a light dark cycle
Physiology & Behavior, 1995Co-Authors: Michaela Hau, Eberhard GwinnerAbstract:Circadian rhythms of most passerine birds and some reptiles depend to a considerable extent on the presence of periodic melatonin, which is considered to be part of the central pacemaking system. Recent results on house sparrows have suggested that melatonin, apart from its periodic effects on the circadian system, may also exert effects derived from a constant action: Continuous melatonin applied through subcutaneously implanted silastic tubing enhanced the synchronization response to a low-amplitude light-dark Zeitgeber, indicating some kind of sensitization to Zeitgeber stimuli. In the present study we tested the prediction derived from these results, that speed of resynchronization after a phase shift of a light-cycle should be enhanced if melatonin is continuously administered. We found that, indeed, house sparrows required less time to resynchronize to an 8 h advance or delay phase shift of a low-amplitude light-dark cycle while carrying a silastic implant filled with melatonin than while carrying an empty implant. The effect is suggested to result from either one or a combination of the following mechanisms: (i) An increased circadian visual sensitivity, (ii) a diminished amplitude of the circadian oscillation, iii) an altered feedback of the locomotor activity to the oscillatory system.
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Circadian entrainment by feeding cycles in house sparrows, Passer domesticus.
Journal of comparative physiology. A Sensory neural and behavioral physiology, 1992Co-Authors: Michaela Hau, Eberhard GwinnerAbstract:We studied the potential Zeitgeber qualities of periodic food availability on the circadian rhythms of locomotor and feeding activity of house sparrows. The birds were initially held in a LD-cycle of 12:12 h, with food restricted to the light phase. After transfer to constant dim light, the birds remained entrained by the restricted feeding schedule. Following an exposure to food ad libitum conditions, the rhythms could be re-synchronized by the feeding cycle. Shortening of the Zeitgeber period to 23.5 h resulted in the loss of entrainment in most birds, whereas a longer Zeitgeber period of 25 h re-entrained the rhythms of most birds. Although these results prove that periodic food availability can act as a Zeitgeber for the circadian rhythms of house sparrows, several features of our data indicate that restricted feeding is only a weak Zeitgeber. The pattern of feeding activity prior to the daily time of food access shown under some experimental conditions suggests that anticipation is due to a positive phase-angle difference of the birds' normal circadian system rather than being caused by a separate pacemaker.
Hanspeter Herzel - One of the best experts on this subject based on the ideXlab platform.
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conceptual models of entrainment jet lag and seasonality
Frontiers in Physiology, 2020Co-Authors: Isao T. Tokuda, Christoph Schmal, Bharath Ananthasubramaniam, Hanspeter HerzelAbstract:Understanding entrainment of circadian rhythms is a central goal of chronobiology. Many factors, such as period, amplitude, Zeitgeber strength, and daylength, govern entrainment ranges and phases of entrainment. We have tested whether simple amplitude-phase models can provide insight into the control of entrainment phases. Using global optimization, we derived conceptual models with just three free parameters (period, amplitude, and relaxation rate) that reproduce known phenotypic features of vertebrate clocks: phase response curves (PRCs) with relatively small phase shifts, fast re-entrainment after jet lag, and seasonal variability to track light onset or offset. Since optimization found multiple sets of model parameters, we could study this model ensemble to gain insight into the underlying design principles. We found complex associations between model parameters and entrainment features. Arnold onions of representative models visualize strong dependencies of entrainment on periods, relative Zeitgeber strength, and photoperiods. Our results support the use of oscillator theory as a framework for understanding the entrainment of circadian clocks.
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Conceptual models of entrainment, jet-lag, and seasonality
2019Co-Authors: Isao T. Tokuda, Christoph Schmal, Bharath Ananthasubramaniam, Hanspeter HerzelAbstract:Understanding entrainment of circadian rhythms is a central goal of chronobiology. Many factors, such as period, amplitude, Zeitgeber strength, and day-length, govern entrainment ranges and the phase of entrainment. Using global optimization, we derive conceptual models with just three free parameters (period, amplitude, relaxation rate) that reproduce known phenotypic features of vertebrate clocks: relatively small phase response curves (PRCs), fast re-entrainment after jet-lag, and seasonal variability to track light onset or offset. Since optimization found multiple sets of model parameters, we can study this model ensemble to gain insight into the underlying design principles. We find that amplitudes control the size of PRCs, that fast relaxation supports short jet-lag, and that specific periods allow reasonable seasonal phase shifts. Arnold onions of representative models visualize strong dependencies of entrainment on periods, relative Zeitgeber strength, and photoperiod.
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Tuning the phase of circadian entrainment.
Journal of the Royal Society Interface, 2015Co-Authors: Grigory Bordyugov, Adrián E. Granada, Achim Kramer, Ute Abraham, Pia Rose, Katharina Imkeller, Hanspeter HerzelAbstract:The circadian clock coordinates daily physiological, metabolic and behavioural rhythms. These endogenous oscillations are synchronized with external cues ('Zeitgebers'), such as daily light and temperature cycles. When the circadian clock is entrained by a Zeitgeber, the phase difference ψ between the phase of a clock-controlled rhythm and the phase of the Zeitgeber is of fundamental importance for the fitness of the organism. The phase of entrainment ψ depends on the mismatch between the intrinsic period τ and the Zeitgeber period T and on the ratio of the Zeitgeber strength to oscillator amplitude. Motivated by the intriguing complexity of empirical data and by our own experiments on temperature entrainment of mouse suprachiasmatic nucleus (SCN) slices, we present a theory on how clock and Zeitgeber properties determine the phase of entrainment. The wide applicability of the theory is demonstrated using mathematical models of different complexity as well as by experimental data. Predictions of the theory are confirmed by published data on Neurospora crassa strains for different period mismatches τ - T and varying photoperiods. We apply a novel regression technique to analyse entrainment of SCN slices by temperature cycles. We find that mathematical models can explain not only the stable asymptotic phase of entrainment, but also transient phase dynamics. Our theory provides the potential to explore seasonal variations of circadian rhythms, jet lag and shift work in forthcoming studies.
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A theoretical study on seasonality.
Frontiers in neurology, 2015Co-Authors: Christoph Schmal, Hanspeter Herzel, Jihwan Myung, Grigory BordyugovAbstract:Additionally to being endogenous, a circadian system must be able to communicate with the outside world and align its rhythmicity to the environment. As a result of such alignment, external Zeitgebers can entrain the circadian system. Entrainment expresses itself in coinciding periods of the circadian oscillator and the Zeitgeber and a stationary phase difference between them. The range of period mismatches between the circadian system and the Zeitgeber that Zeitgeber can overcome to entrain the oscillator is called an entrainment range. The width of the entrainment range usually increases with increasing Zeitgeber strength, resulting in a wedge-like Arnold tongue. This classical view of entrainment does not account for the effects of photoperiod on entrainment. Zeitgebers with extremely small or large photoperiods are intuitively closer to constant environments than equinoctial Zeitgebers and hence are expected to produce a narrower entrainment range. In this paper, we present theoretical results on entrainment under different photoperiods. We find that in the photoperiod-detuning parameter plane, the entrainment zone is shaped in the form of a skewed onion. The bottom and upper points of the onion are given by the free running periods in DD and LL, respectively. The widest entrainment range is found near photoperiods of 50%. Within the onion, we calculated the entrainment phase that varies over a range of 12h. The results of our theoretical study explain the experimentally observed behaviour of the entrainment phase in dependence on the photoperiod.
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Human chronotypes from a theoretical perspective.
PloS one, 2013Co-Authors: Adrián E. Granada, Grigory Bordyugov, Achim Kramer, Hanspeter HerzelAbstract:The endogenous circadian timing system has evolved to synchronize an organism to periodically recurring environmental conditions. Those external time cues are called Zeitgebers. When entrained by a Zeitgeber, the intrinsic oscillator adopts a fixed phase relation ψ to the Zeitgeber. Here, we systematically study how the phase of entrainment depends on clock and Zeitgeber properties. We combine numerical simulations of amplitude-phase models with predictions from analytically tractable models. In this way we derive relations between the phase of entrainment ψ to the mismatch between the endogenous and Zeitgeber period, the Zeitgeber strength, and the range of entrainment. A core result is the "180° rule" asserting that the phase ψ varies over a range of about 180° within the entrainment range. The 180° rule implies that clocks with a narrow entrainment range ("strong oscillators") exhibit quite flexible entrainment phases. We argue that this high sensitivity of the entrainment phase contributes to the wide range of human chronotypes.
Menno P. Gerkema - One of the best experts on this subject based on the ideXlab platform.
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Concurrent decrease of vasopressin and protein kinase Cα immunoreactivity during the light phase in the vole suprachiasmatic nucleus
Neuroscience Letters, 1998Co-Authors: K Jansen, Menno P. GerkemaAbstract:Vasopressin (AVP) is a major neuropeptide in the suprachiasmatic nucleus, the mammalian hypothalamic circadian pacemaker. Protein kinase C alpha is a putatively coupled intracellular messenger. Mean numbers of AVP- and protein kinase C alpha- immunoreactive neurons were determined in the suprachiasmatic nucleus of common voles, entrained to a 12:12 h light-dark (LD) cycle, at the beginning of the light period (Zeitgeber time zero) and 6 h later (Zeitgeber time six). At Zeitgeber time zero, mean numbers of AVP- and protein kinase C alpha- immunoreactive neurons were 2194 and 9897, respectively. Both numbers decreased significantly with about 40% at Zeitgeber time six. This concurrent decrease was most pronounced in the dorsomedial aspect of the suprachiasmatic nucleus. These findings are consistent with the findings of a peak of AVP release in rats during the early light phase. (C) 1998 Published by Elsevier Science Ireland Ltd. All rights reserved
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Concurrent decrease of vasopressin and protein kinase Cα immunoreactivity during the light phase in the vole suprachiasmatic nucleus
Neuroscience letters, 1998Co-Authors: K Jansen, Eddy A. Van Der Zee, Menno P. GerkemaAbstract:Vasopressin (AVP) is a major neuropeptide in the suprachiasmatic nucleus, the mammalian hypothalamic circadian pacemaker. Protein kinase Calpha is a putatively coupled intracellular messenger. Mean numbers of AVP- and protein kinase Calpha-immunoreactive neurons were determined in the suprachiasmatic nucleus of common voles, entrained to a 12:12 h light-dark (LD) cycle, at the beginning of the light period (Zeitgeber time zero) and 6 h later (Zeitgeber time six). At Zeitgeber time zero, mean numbers of AVP- and protein kinase Calpha- immunoreactive neurons were 2194 and 9897, respectively. Both numbers decreased significantly with about 40% at Zeitgeber time six. This concurrent decrease was most pronounced in the dorsomedial aspect of the suprachiasmatic nucleus. These findings are consistent with the findings of a peak of AVP release in rats during the early light phase.
Konstantin V. Danilenko - One of the best experts on this subject based on the ideXlab platform.
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is sleep per se a Zeitgeber in humans
Journal of Biological Rhythms, 2003Co-Authors: Konstantin V. Danilenko, Christian Cajochen, Anna WirzjusticeAbstract:It is not clear whether shifting of sleep per se, without a concomitant change in the light-dark cycle, can induce a phase shift of the human circadian pacemaker. Two 9-day protocols (crossover, counterbalanced order) were com- pleted by 4 men and 6 women (20-34 years) after adherence to a 2330 to 0800 h sleep episode at home for 2 weeks. Following a modified baseline constant routine (CR) protocol on day 2, they remained under continuous near-darkness (< 0.2 lux, including sleep) for 6 days. Four isocaloric meals were equally distrib- uted during scheduled wakefulness, and their timing was held constant. Sub- jects remained supine in bed from 2100 to 0800 h on all days; sleep was fixed from 2330 to 0800 h in the control condition and was gradually advanced 20 min per day during the sleep advance condition until a 2-h difference had been attained. On day 9, a 25 to 27 h CR protocol (~0.1 lux) was carried out. Phase markers were the evening decline time of the core body temperature (CBT) rhythm and sali- vary melatonin onset (3 pg/ml threshhold). In the fixed sleep condition, the phase drift over 7 days ranged from +1.62 to -2.56 h (for both CBT and melatonin rhythms, which drifted in parallel). The drifts were consistently advanced in the sleep advance schedule by +0.66 ± 0.23 (SEM) h for CBT (p = 0.02) and by 0.27 ± 0.14 h for melatonin rhythms (p = 0.09). However, this advance was small to medium according to effect size. Sleep per se may feed back onto the circadian pacemaker, but it appears to be a weak Zeitgeber in humans.
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Is sleep per se a Zeitgeber in humans
Journal of biological rhythms, 2003Co-Authors: Konstantin V. Danilenko, Christian Cajochen, Anna Wirz-justiceAbstract:It is not clear whether shifting of sleep per se, without a concomitant change in the light-dark cycle, can induce a phase shift of the human circadian pacemaker. Two 9-day protocols (crossover, counterbalanced order) were completed by 4 men and 6 women (20-34 years) after adherence to a 2330 to 0800 h sleep episode at home for 2 weeks. Following a modified baseline constant routine (CR) protocol on day 2, they remained under continuous near-darkness (< 0.2 lux, including sleep) for 6 days. Four isocaloric meals were equally distributed during scheduled wakefulness, and their timing was held constant. Subjects remained supine inbed from 2100 to 0800 h on all days; sleep was fixed from 2330 to 0800 h in the control condition and was gradually advanced 20 min per day during the sleep advance condition until a 2-h difference had been attained. On day 9, a 25 to 27 h CR protocol (approximately 0.1 lux) was carried out. Phase markers were the evening decline time of the core body temperature (CBT) rhythm and salivary melatonin onset (3 pg/ml threshhold). In the fixed sleep condition, the phase drift over 7 days ranged from +1.62 to -2.56 h (for both CBT and melatonin rhythms, which drifted in parallel). The drifts were consistently advanced in the sleep advance schedule by +0.66 +/- 0.23 (SEM) h for CBT (p = 0.02) and by 0.27 +/- 0.14 h for melatonin rhythms (p = 0.09). However, this advance was small to medium according to effect size. Sleep per se may feed back onto the circadian pacemaker, but it appears to be a weak Zeitgeber in humans.
