Effects of early weaning on the circadian rhythm and behavioral satiety sequence in rats

The objective of this work was to study the effect of early weaning on circadian rhythm and the behavioral satiety sequence in adult rats. Male Wistar rat pups were weaned for separation from the mother at 15 (D15), 21 (D21) and 30 (D30) days old. Body weight and food intake was measured every 30 days until pups were 150 days old. At 90 days of age, the circadian rhythm of food intake was evaluated every 4 h for three days. Behavioral satiety was evaluated at 35 and 100 days of age. This work demonstrated that body weight and food intake were not altered, but the behavioral satiety sequence demonstrated that the D15 group delayed satiety compared with the D30 group at 100 days of age. In the circadian rhythm of the food intake study, early weaning (D15) changed food intake in the intermediary period of the light phase and in the intermediary period of the dark phase. In conclusion, our study showed that early weaning may alter the feeding behavior mainly in relation to satiety and the circadian rhythm of feeding. It is possible that the presence of other environmental stimuli during early weaning can cause hyperphagia and deregulate the mechanisms of homeostasis and body weight control. This study supports theories that depict insults during early life as determinants of chronic diseases.     

Circadian inputs influence the performance of a spiking, movement-sensitive neuron in the visual system of the blowfly.

Long-term extracellular recordings from a spiking, movement-sensitive giant neuron (H1) in the third optic ganglion of the blowfly Calliphora vicina (L.) revealed periodic endogenous sensitivity fluctuations. The sensitivity changes showed properties typical of an endogenous circadian rhythm. This was true for the responses in reaction to intensity changes of visual patterns as well as for the responses elicited by pattern movement. For these two types of stimuli, the circadian fluctuations were comparable, but the envelope in the case of responses to movement was more robust. A circadian fluctuation in responses to movement is, therefore, present at the level of single elementary movement detectors. The tonic activity of the neuron was also shown to be under circadian control. In constant darkness (DD) the fluctuation was circadian, whereas in constant light it was not. The subjective light-dark (LD) transitions in the tonic activity in DD closely followed the LD transitions in the holding cages initially; that is, there was low activity at night and high activity during the daytime. The sensitivity fluctuations in response to visual stimuli led the tonic spike activity fluctuations by several hours.     

Maternal influence on activity rhythms and reproductive development in Djungarian hamster pups

Photoperiodism and entrainment of the circadian rhythm of locomotor activity were investigated in juvenile Djungarian hamsters. Animals were housed in simulated burrows. Activity was measured as the animal's emergence from a dark nest chamber into an outer box exposed to the room illumination. This burrow emergence activity exhibited marked circadian rhythmicity. Interactions between mother hamsters and their offspring were examined in the simulated burrow system. Male reproductive responses were determined by measuring testicular weights at the time of weaning. It was shown that photoperiodic information received between Days 1 and 15 of life failed to alter the rate of testicular development, but that after Day 15 testicular growth was photoperiod-dependent. The mother, when entrained to a long photoperiod, did not influence the photoperiodic responses of her pups when they were confined to a dark nest box. In contrast, the mother did influence the circadian entrainment patterns of her pups. Pups exhibited a well-developed circadian activity rhythm at weaning with a phase angle roughly similar to that of the mother's activity rhythm. When the maternal rhythms were discrepant with photoperiod information received by the pups directly from the environment, the pups' activity rhythms were synchronized with the light/dark cycle rather than with the rhythm of their mother. Thus, it appears that although pups may first become entrained by maternal cues, they rapidly adjust to the environmental light cycle after leaving the nest.     

Effect of stress and dexamethasone treatment on circadian rhythms of melatonin and corticosterone in ring dove (Streptopelia risoria)

The possible relationship between the circadian rhythm of blood levels of melatonin and corticosterone in ring dove (Streptopelia risoria) subjected to both immobilization stress and immobilization stress plus dexamethasone treatment were studied. The results show changes in the circadian rhythm of melatonin, with increased day-time levels in situations of stress accompanied by increased corticosterone levels. The highest blood melatonin levels over the 24 h of the study were obtained when the animals were treated with dexamethasone and then subjected to stress. Given the antioxidant role of melatonin, our results support the idea of melatonin-corticosterone coupling with the possibility that melatonin released in situations of stress counteracts the adverse effects of glucocorticoids on the organism.     

Effects of angiotensins on day-night fluctuations and stress-induced changes in blood pressure

In this study we evaluated by telemetry the effects of ANG II and ANG-(1-7) infusion on the circadian rhythms of blood pressure (BP) and heart rate (HR) and on the cardiovascular adjustment resulting from restraint stress in rats. ANG II or ANG-(1-7) or vehicle were infused subcutaneously for 7 days. Restraint stress was carried out before, during, and after infusion at 7-day intervals. Parallel with an increase in MAP, ANG II infusion produced an inversion of MAP circadian rhythm with a significant MAP acrophase inversion. It also produced bradycardia during the first 3 days of infusion. Thereafter, HR progressively increased, reaching values similar to or above those of the control period at the end of the infusion period. HR circadian variation was not changed by ANG II infusion. Strikingly, ANG II significantly attenuated the increase in MAP induced by restraint stress without altering the HR response. ANG-(1-7) infusion produced a slight but significant decrease in MAP restricted to the daytime period. No significant changes in the MAP acrophase were observed. In addition, ANG-(1-7) infusion produced a small but significant sustained bradycardia. ANG-(1-7) did not change cardiovascular responses to restraint stress. These data indicate that ANG II can influence the activity of brain areas involved in the determination of stress-induced or circadian-dependent variations of blood pressure without changing HR fluctuations. A significant modulatory influence of ANG-(1-7) on basal MAP and HR is also suggested.     

Period responses to Zeitgeber signals stabilize circadian clocks during entrainment

Circadian clocks with characteristic period (τ) can be entrained to light/dark (LD) cycles by means of (i) phase shifts which are due to D/L “dawn” and/or L/D “dusk” transitions, (ii) period changes associated with long-term light exposure, or (iii) by combinations of the above possibilities. Based on stability analysis of a model circadian clock it was predicted that nocturnal burrowing mammals would benefit less from period responses than their diurnal counterparts. The model further predicted that maximal stability of circadian clock is reached when the clock slightly changes both its phase and period in response to light stimuli. Analyses of empirical phase response curve (PRC) and period response curve (τRC) of some diurnal and nocturnal mammals revealed that PRCs of both diurnal and nocturnal mammals have similar waveform while τRCs of nocturnal mammals are of smaller amplitude than those of diurnal mammals. The shape of the τRC also changes with age and with increasing strength of light stimuli. During erratic fluctuations in light intensity under different weather conditions, the stability of phase of entrainment of circadian clocks appears to be achieved by an interplay between phase and period responses and the strength of light stimuli.     

The Circadian Rhythm of Stomatal Opening: Evidence for the Involvement of Potassium and Chloride Fluxes

Epidermal strips were taken from plants of Commelina communisat opposite phases of an entrained circadian rhythm, at timeswhich corresponded to the middle of the day phase and the middleof the night phase. Earlier observations of much reduced openingin response to external stimuli in the night phase were confirmed. Evidence is now presented that uptake or retention of specificions by the guard cells is affected by the phase of the rhythm.The overt expression of the rhythm was reduced if the epidermiswas presented with potassium iminodiacetate instead of potassiumchloride. Iminodiacetate is a non-absorbable anion, and thisresult suggested that chloride uptake is important for the fullexpression of the rhythm. The use of an anion channel inhibitor(DIDS) gave strong support for this conclusion. Reduced uptakeof chloride would explain an earlier observation that formationof malate is greater during the night phase. When the guard cells were presented with sodium chloride insteadof potassium chloride there was still an overt expression ofthe rhythm, but this was of reduced magnitude. This suggeststhat potassium movements might contribute to the rhythm, andstudies of ionic fluxes using ⁸⁶Rb as a tracer showed greatereffluxes during the night phase. It is concluded that the circadian rhythm in stomatal openingmay be the result of a varying ability of the guard cells toaccumulate or retain both chloride and potassium ions. Key words: Circadian rhythm, stomatal opening, Commelina communis, potassium and chloride fluxes     

The Drosophila dCREB2 gene affects the circadian clock

We report the role of dCREB2, the Drosophila homolog of CREB/CREM, in circadian rhythms. dCREB2 activity cycles with a 24 hr rhythm in flies, both in a light:dark cycle and in constant darkness. A mutation in dCREB2 shortens circadian locomotor rhythm in flies and dampens the oscillation of period, a known clock gene. Cycling dCREB2 activity is abolished in a period mutant, indicating that dCREB2 and Period affect each other and suggesting that the two genes participate in the same regulatory feedback loop. We propose that dCREB2 supports cycling of the Period/Timeless oscillator. These findings support CREB's role in mediating adaptive behavioral responses to a variey of environmental stimuli (stress, growth factors, drug addiction, circadian rhythms, and memory formation) in mammals and long-term memory formation and circadian rhythms in Drosophila.     

Circadian Modulation of Acute Alcohol Sensitivity But Not Acute Tolerance in Drosophila

An increased understanding of the factors affecting behavioral and neurological responses to alcohol and alcohol physiology is necessary given the tremendous toll alcohol abuse and alcoholism exert on individuals and society. At the behavioral and molecular levels, the response to alcohol appears remarkably conserved from Drosophila to humans, suggesting that investigations across model species can provide insight into the identification of common modulatory factors. We investigated the interaction between the circadian clock and alcohol sensitivity, alcohol tolerance, and alcohol absorbance in Drosophila melanogaster. Using a loss-of-righting reflex (LoRR) assay, we found that flies exhibit a circadian rhythm in the LoRR, with the greatest sensitivity to alcohol occurring from mid to late night, corresponding to the flies' inactive phase. As predicted, a circadian rhythm in the LoRR was absent in circadian mutant flies and under conditions in which the circadian clock was nonfunctional. Circadian modulation of the response to alcohol was not due to circadian regulation of alcohol absorbance. Similar to other animals, Drosophila develop acute and chronic tolerance to alcohol upon repeat exposures. We found that the circadian clock did not modulate the development of acute alcohol tolerance measured as the difference in sensitivity to alcohol between naïve and pre-exposed flies. Thus, the circadian clock modulates some, but not all, of the behavioral responses to alcohol exposure, suggesting that specific mechanisms underlie the observed circadian modulation of LoRR rather than global cellular circadian regulation. This study provides valuable new insights in our understanding of the circadian modulation of alcohol-induced behaviors that ultimately could facilitate preventative measures in combating alcohol abuse and alcoholism. (Author correspondence: lyons@bio.fsu.edu)     

Circadian modulation of acute alcohol sensitivity but not acute tolerance in Drosophila

An increased understanding of the factors affecting behavioral and neurological responses to alcohol and alcohol physiology is necessary given the tremendous toll alcohol abuse and alcoholism exert on individuals and society. At the behavioral and molecular levels, the response to alcohol appears remarkably conserved from Drosophila to humans, suggesting that investigations across model species can provide insight into the identification of common modulatory factors. We investigated the interaction between the circadian clock and alcohol sensitivity, alcohol tolerance, and alcohol absorbance in Drosophila melanogaster. Using a loss-of-righting reflex (LoRR) assay, we found that flies exhibit a circadian rhythm in the LoRR, with the greatest sensitivity to alcohol occurring from mid to late night, corresponding to the flies' inactive phase. As predicted, a circadian rhythm in the LoRR was absent in circadian mutant flies and under conditions in which the circadian clock was nonfunctional. Circadian modulation of the response to alcohol was not due to circadian regulation of alcohol absorbance. Similar to other animals, Drosophila develop acute and chronic tolerance to alcohol upon repeat exposures. We found that the circadian clock did not modulate the development of acute alcohol tolerance measured as the difference in sensitivity to alcohol between na?ve and pre-exposed flies. Thus, the circadian clock modulates some, but not all, of the behavioral responses to alcohol exposure, suggesting that specific mechanisms underlie the observed circadian modulation of LoRR rather than global cellular circadian regulation. This study provides valuable new insights in our understanding of the circadian modulation of alcohol-induced behaviors that ultimately could facilitate preventative measures in combating alcohol abuse and alcoholism.     

Circadian changes in central excitability—the origin of behavioural rhythms in tsetse flies and other animals?

Diel changes in seven behavioural responses of male tsetse flies, G.morsitans, were measured under controlled conditions in the laboratory. These responses involved different motor and sensory modalities, and were evoked by stimuli that were either exogenously applied at regular intervals or were ever-present. Six (possibly seven) of the responses were modulated across the photophase (of LD 12 : 12) in the V-pattern typical of biting behaviour in the field: morning and evening responses were greatest, noon least. Two of these rhythms were shown to persist in constant conditions, indicating that the underlying control is truly circadian. Comparison of the form of an optokinetic response rhythm at high and low stimulus intensities, implied central control of the response modulation. This, plus the close similarity in phase of the different rhythms, is interpreted as indicating circadian changes in central excitatory state (arousal) as the underlying neurophysiological basis of behavioural rhythmicity. It is suggested that the sarrie arousal system controls the comparable, longer and shorter term changes in excitability that result from starvation, sex, etc.     

The temporal organization of daily torpor and hibernation: circadian and circannual rhythms

Mammals and birds have evolved the ability to maintain a high and constant body temperature T_b over a wide range of ambient temperatures T_a using endogenous heat production. In many, especially small endotherms, cost for thermoregulatory heat production can exceed available energy; to overcome these energetic bottlenecks, they enter a state of torpor (a regulated reduction of T_b and metabolic rate). Since the occurrence of torpor in many species is a seasonal event and occurs at certain times of the day, we review whether circadian and circannual rhythms, important in the timing of biological events in active animals, also play an important role during torpor when T_b is reduced substantially and may even fall below 0°C. The two distinct patterns of torpor, hibernation (prolonged torpor) and daily torpor, differ substantially in their interaction with the circadian system. Daily torpor appears to be integrated into the normal circadian rhythm of activity and rest, although torpor is not restricted only to the normal rest phase of an animal. In contrast, hibernation can last for several days or even weeks, although torpor never spans the entire hibernation season, but is interrupted by periodic arousals and brief normothermic periods. Clearly, a day is no longer divided in activity and rest, and at first glance the role of the circadian system appears negligible. However, in several hibernators, arousals not only follow a regular pattern consistent with a circadian rhythm, but also are entrainable by external stimuli such as photoperiod and T_a. The extent of the interaction between the circadian and circannual system and hibernation varies among species. Biological rhythms of hibernators for which food availability appears to be predictable seasonally and that hibernate in deep and sealed burrows show little sensitivity to external stimuli during hibernation and hence little entrainability of arousal events. In contrast, opportunistic hibernators, which some times use arousals for foraging and hibernate in open and accessible hibernacula, are susceptible to external zeitgebers. In opportunistic hibernators, the circadian system plays a major role in maintaining synchrony between the normal day-night cycle and occasional foraging. Although the daily routine of activity and rest is abandoned during hibernation, the circadian system appears to remain functional, and there is little evidence it is significantly affected by low T_b. (Chronobiology International, 17(2), 103-128, 2000)     

The circadian timing system and reproduction in mammals.

Circadian systems in a wide variety of organisms all appear to include three basic components: 1) biological oscillators that maintain a self-sustained circadian periodicity in the absence of environmental time cues; 2) input pathways that convey environmental information, especially light cues, that can entrain the circadian oscillations to local time; and 3) output pathways that drive overt circadian rhythms, such as the rhythms of locomotor activity and a variety of endocrine rhythms. In mammals, the circadian system is employed in the regulation of reproductive physiology and behavior in two very important ways. 1) In some species, there is a strong circadian component in the timing of ovulation and reproductive behavior, ensuring that these events will occur at a time when the animal is most likely to encounter a potential mate. 2) Many mammals exhibit seasonal reproductive rhythms that are largely under photoperiod regulation; in these species, the circadian system and the pineal gland are crucial components of the mechanism that is used to measure day length. The rhythm of pineal melatonin secretion is driven by a neural pathway that includes the circadian oscillator(s) in the suprachiasmatic nuclei. Melatonin is secreted at night in all mammals, and the duration of each nocturnal episode of melatonin secretion is inversely related to day length. The pineal melatonin rhythm appears to serve as an internal signal that represents day length and that is capable of regulating a variety of seasonal variations in physiology and behavior.     

Social influences on mammalian circadian rhythms: animal and human studies

While light is considered the dominant stimulus for entraining (synchronizing) mammalian circadian rhythms to local environmental time, social stimuli are also widely cited as 'zeitgebers' (time-cues). This review critically assesses the evidence for social influences on mammalian circadian rhythms, and possible mechanisms of action. Social stimuli may affect circadian behavioural programmes by regulating the phase and period of circadian clocks (i.e. a zeitgeber action, either direct or by conditioning to photic zeitgebers), by influencing daily patterns of light exposure or modulating light input to the clock, or by associative learning processes that utilize circadian time as a discriminative or conditioned stimulus. There is good evidence that social stimuli can act as zeitgebers. In several species maternal signals are the primary zeitgeber in utero and prior to weaning. Adults of some species can also be phase shifted or entrained by single or periodic social interactions, but these effects are often weak, and appear to be mediated by social stimulation of arousal. There is no strong evidence yet for sensory-specific nonphotic inputs to the clock. The circadian phase-dependence of clock resetting to social stimuli or arousal (the 'nonphotic' phase response curve, PRC), where known, is distinct from that to light and similar in diurnal and nocturnal animals. There is some evidence that induction of arousal can modulate light input to the clock, but no studies yet of whether social stimuli can shift the clock by conditioning to photic cues, or be incorporated into the circadian programme by associative learning. In humans, social zeitgebers appear weak by comparison with light. In temporal isolation or under weak light-dark cycles, humans may ignore social cues and free-run independently, although cases of mutual synchrony among two or more group-housed individuals have been reported. Social cues may affect circadian timing by controlling sleep-wake states, but the phase of entrainment observed to fixed sleep-wake schedules in dim light is consistent with photic mediation (scheduled variations in behavioural state necessarily create daily light-dark cycles unless subjects are housed in constant dark or have no eyes). By contrast, discrete exercise sessions can induce phase shifts consistent with the nonphotic PRC observed in animal studies. The best evidence for social entrainment in humans is from a few totally blind subjects who synchronize to the 24 h day, or to near-24 h sleep-wake schedules under laboratory conditions. However, the critical entraining stimuli have not yet been identified, and there are no reported cases yet of social entrainment in bilaterally enucleated blind subjects. The role of social zeitgebers in mammalian behavioural ecology, their mechanisms of action, and their utility for manipulating circadian rhythms in humans, remains to be more fully elaborated.     

Isochron-based phase response analysis of circadian rhythms

Circadian rhythms possess the ability to robustly entrain to the environmental cycles. This ability relies on the phase synchronization of circadian rhythm gene regulation to different environmental cues, of which light is the most obvious and important. The elucidation of the mechanism of circadian entrainment requires an understanding of circadian phase behavior. This article presents two phase analyses of oscillatory systems for infinitesimal and finite perturbations based on isochrons as a phase metric of a limit cycle. The phase response curve of circadian rhythm can be computed from the results of the analyses. The application to a mechanistic Drosophila circadian rhythm model gives experimentally testable hypotheses for the control mechanisms of circadian phase responses and evidence for the role of phase and period modulations in circadian photic entrainment.     

THE TEMPORAL ORGANIZATION OF DAILY TORPOR AND HIBERNATION: CIRCADIAN AND CIRCANNUAL RHYTHMS

Mammals and birds have evolved the ability to maintain a high and constant body temperature T_b over a wide range of ambient temperatures T_a using endogenous heat production. In many, especially small endotherms, cost for thermoregulatory heat production can exceed available energy; to overcome these energetic bottlenecks, they enter a state of torpor (a regulated reduction of T_b and metabolic rate). Since the occurrence of torpor in many species is a seasonal event and occurs at certain times of the day, we review whether circadian and circannual rhythms, important in the timing of biological events in active animals, also play an important role during torpor when T_b is reduced substantially and may even fall below 0°C. The two distinct patterns of torpor, hibernation (prolonged torpor) and daily torpor, differ substantially in their interaction with the circadian system. Daily torpor appears to be integrated into the normal circadian rhythm of activity and rest, although torpor is not restricted only to the normal rest phase of an animal. In contrast, hibernation can last for several days or even weeks, although torpor never spans the entire hibernation season, but is interrupted by periodic arousals and brief normothermic periods. Clearly, a day is no longer divided in activity and rest, and at first glance the role of the circadian system appears negligible. However, in several hibernators, arousals not only follow a regular pattern consistent with a circadian rhythm, but also are entrainable by external stimuli such as photoperiod and T_a. The extent of the interaction between the circadian and circannual system and hibernation varies among species. Biological rhythms of hibernators for which food availability appears to be predictable seasonally and that hibernate in deep and sealed burrows show little sensitivity to external stimuli during hibernation and hence little entrainability of arousal events. In contrast, opportunistic hibernators, which some times use arousals for foraging and hibernate in open and accessible hibernacula, are susceptible to external zeitgebers. In opportunistic hibernators, the circadian system plays a major role in maintaining synchrony between the normal day-night cycle and occasional foraging. Although the daily routine of activity and rest is abandoned during hibernation, the circadian system appears to remain functional, and there is little evidence it is significantly affected by low T_b. (Chronobiology International, 17(2), 103–128, 2000)     

Circadian Rhythms in Stomatal Responsiveness to Red and Blue Light

Stomata of many plants have circadian rhythms in responsiveness to environmental cues as well as circadian rhythms in aperture. Stomatal responses to red light and blue light are mediated by photosynthetic photoreceptors; responses to blue light are additionally controlled by a specific blue-light photoreceptor. This paper describes circadian rhythmic aspects of stomatal responsiveness to red and blue light in Vicia faba. Plants were exposed to a repeated light:dark regime of 1.5:2.5 h for a total of 48 h, and because the plants could not entrain to this short light:dark cycle, circadian rhythms were able to "free run" as if in continuous light. The rhythm in the stomatal conductance established during the 1.5-h light periods was caused both by a rhythm in sensitivity to light and by a rhythm in the stomatal conductance established during the preceding 2.5-h dark periods. Both rhythms peaked during the middle of the subjective day. Although the stomatal response to blue light is greater than the response to red light at all times of day, there was no discernible difference in period, phase, or amplitude of the rhythm in sensitivity to the two light qualities. We observed no circadian rhythmicity in net carbon assimilation with the 1.5:2.5 h light regime for either red or blue light. In continuous white light, small rhythmic changes in photosynthetic assimilation were observed, but at relatively high light levels, and these appeared to be attributable largely to changes in internal CO2 availability governed by stomatal conductance.     

Circadian phase resetting in response to light-dark and dark-light transitions

Phase shifting of circadian systems by light has been attributed both to parametric effects on angular velocity elicited by a tonic response to the luminance level and to nonparametric instantaneous shifts induced by a phasic response to the dark-light (D>L) and light-dark (L>D) transitions. Claims of nonparametric responses are partly based on "step-PRCs," that is, phase response curves derived from such transitions. Step-PRCs in nocturnal mammals show mostly delays after lights-on and advances after lights-off, and therefore appear incompatible with phase delays generated by light around dusk and advances by light around dawn. We have pursued this paradox with 2 experimental protocols in mice. We first use the classic step-PRC protocol on wheel running activity, using the center of gravity as a phase marker to minimize the masking effects of light. The experiment was done for 3 different light intensities (1, 10, and 100 lux). D>L transitions evoke mostly delays and L>D transitions show no clear tendency to either delay or advance. Overall there is little or no circadian modulation. A 2nd protocol aimed to avoid the problem of masking by assessing phase before and after the light stimuli, both in DD. Light stimuli consisted of either a slow light intensity increase over 48 h followed by abruptly switching off the light, or an abrupt switch on followed by a slow decrease toward total darkness during 48 h. If the abrupt transitions were responsible for phase shifting, we expected large differences between the 2 stimuli. Both light stimuli yielded similar PRCs characterized by delays only with circadian modulation. The results can be adequately explained by a model in which all PRCs evoked by steps result in fact from tonic responses to the light following a step-up or preceding a step-down. In this model only the response reduction of tonic velocity change after the 1st hour is taken into account. The data obtained in both experiments are thus compatible with tonic velocity responses. Contrary to standard interpretation of step-PRCs, nonparametric responses to the transitions are unlikely since they would predict delays in response to lights-off, advances in response to lights-on, while the opposite was found. Although such responses cannot be fully excluded, parsimony does not require invocation of a role for transitions, since all the data can readily be explained by tonic velocity (parametric) effects, which must exist because of the dependence of tau on light intensity.     

The timing of the human circadian clock is accurately represented by the core body temperature rhythm following phase shifts to a three-cycle light stimulus near the critical zone

A double-stimulus experiment was conducted to evaluate the phase of the underlying circadian clock following light-induced phase shifts of the human circadian system. Circadian phase was assayed by constant routine from the rhythm in core body temperature before and after a three-cycle bright-light stimulus applied near the estimated minimum of the core body temperature rhythm. An identical, consecutive three-cycle light stimulus was then applied, and phase was reassessed. Phase shifts to these consecutive stimuli were no different from those obtained in a previous study following light stimuli applied under steady-state conditions over a range of circadian phases similar to those at which the consecutive stimuli were applied. These data suggest that circadian phase shifts of the core body temperature rhythm in response to a three-cycle stimulus occur within 24 h following the end of the 3-day light stimulus and that this poststimulus temperature rhythm accurately reflects the timing of the underlying circadian clock.