Accuracy of human circadian entrainment under natural light conditions: model simulations

The patterns of light intensity to which humans expose their circadian pacemakers in daily life are very irregular and vary greatly from day to day. The circadian pacemaker can adjust to such irregular exposure patterns by daily phase shifts, such as summarized in a phase response curve. It is demonstrated in this paper on the basis of computer simulations applying actually recorded human light exposure patterns that the pacemaker can substantially improve its accuracy by an additional response to light: For that purpose, it should additionally change its angular velocity (and consequently its period tau) in response to light. Reductions of tau in response to light in the morning and increases of tau in response to light in the evening can lead to an increase in entrained pacemaker accuracy with about 25%. Circadian pacemakers have evolved as accurate internal representations of external time, and investigated diurnal mammals all seem to respond to light by changing the period of their circadian pacemaker (in addition to shifting phase). The authors suggest that also human circadian systems take advantage of this possibility and that their pacemakers respond to light by shifting phase and changing period. As a consequence of this postulated mechanism, the simulations demonstrate that the period of the pacemaker under normally entrained conditions is 24 h. The maximum accuracy corresponds to a day-to-day standard deviation of the time of phase 0 of circa 15 min. This is considerably more accurate than the light signal humans usually perceive.     

Exogenous melatonin reduces the resynchronization time after phase shifts of a nonphotic zeitgeber in the house sparrow (Passer domesticus)

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.     

In search of the pathways for light-induced pacemaker resetting in the suprachiasmatic nucleus

Within the suprachiasmatic nucleus (SCN) of the mammalian hypothalamus is a circadian pacemaker that functions as a clock. Its endogenous period is adjusted to the external 24-h light-dark cycle, primarily by light-induced phase shifts that reset the pacemaker's oscillation. Evidence using a wide variety of neurobiological and molecular genetic tools has elucidated key elements that comprise the visual input pathway for SCN photoentrainment in rodents. Important questions remain regarding the intracellular signals that reset the autoregulatory molecular loop within photoresponsive cells in the SCN's retino-recipient subdivision, as well as the intercellular coupling mechanisms that enable SCN tissue to generate phase shifts of overt behavioral and physiological circadian rhythms such as locomotion and SCN neuronal firing rate. Multiple neurotransmitters, protein kinases, and photoinducible genes add to system complexity, and we still do not fully understand how dawn and dusk light pulses ultimately produce bidirectional, advancing and delaying phase shifts for pacemaker entrainment.     

Experimental evidence for a non-clock role of the circadian system in spider mite photoperiodism

In the spider mite Tetranychus urticae photoperiodic time measurement proceeds accurately in orange-red light of 580 nm and above in light/dark cycles with a period length of 20 h but not in 'natural' cycles with a period length of 24 h. To explain these results it is hypothesized that the photoperiodic clock in the spider mite is sensitive to orange-red light, but the Nanda-Hamner rhythm (a circadian rhythm with a free-running period tau of 20 h involved in the photoperiodic response) is not and consequently free runs in orange-red light. To test this hypothesis a zeitgeber was sought that could entrain the Nanda-Hamner rhythm to a 24-h cycle without inducing diapause itself, in order to manipulate the rhythm independently from the orange-red sensitive photoperiodic clock. A suitable zeitgeber was found to be a thermoperiod with a 12-h warm phase and a 12-h cold phase. Combining the thermoperiod with the long-night orange-red light/dark regime, both with a cycle length of 24 h, resulted in a high diapause incidence, although neither regime was capable of inducing diapause on its own. The conclusion is that the Nanda-Hamner rhythm is necessary for the realization of the photoperiodic response, but is not part of the photoperiodic clock, because photoperiodic time measurement takes place in orange-red light whereas the rhythm is not able to 'see' the orange-red light. It is speculated that the Nanda-Hamner rhythm is involved in the timely synthesis of a substrate for the photoperiodic clock in the spider mite.     

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.     

Circadian entrainment by feeding cycles in house sparrows,Passer domesticus

Summary 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.     

Aging alters circadian and light-induced expression of clock genes in golden hamsters

Aging alters numerous aspects of circadian biology, including the amplitude of rhythms generated by the suprachiasmatic nuclei (SCN) of the hypothalamus, the site of the central circadian pacemaker in mammals, and the response of the pacemaker to environmental stimuli such as light. Although previous studies have described molecular correlates of these behavioral changes, to date only 1 study in rats has attempted to determine if there are age-related changes in the expression of genes that comprise the circadian clock itself. We used in situ hybridization to examine the effects of age on the circadian pattern of expression of a subset of the genes that comprise the molecular machinery of the circadian clock in golden hamsters. Here we report that age alters the 24-h expression profile of Clock and its binding partner Bmal1 in the hamster SCN. There is no effect of age on the 24-h profile of either Per1 or Per2 when hamsters are housed in constant darkness. We also found that light pulses, which induce smaller phase shifts in old animals than in young, lead to decreased induction of Per1, but not of Per2, in the SCN of old hamsters.     

Peripheral circadian oscillators in mammals: time and food

Peripheral cells from mammalian tissues, while perfectly capable of circadian rhythm generation, are not light sensitive and thus have to be entrained by nonphotic cues. Feeding time is the dominant zeitgeber for peripheral mammalian clocks: Daytime feeding of nocturnal laboratory rodents completely inverts the phase of circadian gene expression in many tissues, including liver, heart, kidney, and pancreas, but it has no effect on the SCN pacemaker. It is thus plausible that in intact animals, the SCN synchronizes peripheral docks primarily through temporal feeding patterns that are imposed through behavioral rest-activity cycles. In addition, body temperature rhythms, which are themselves dependent on both feeding patterns and rest-activity cycles, can sustain circadian, clock gene activity in vivo and in vitro. The SCN may also influence the phase of rhythmic gene expression in peripheral tissues through direct chemical pathways. In fact, many chemical signals induce circadian gene expression in tissue culture cells. Some of these have been shown to elicit phase shifts when injected into intact animals and are thus candidates for physiologically relevant timing cues. While the response of the SCN to light is strictly gated to respond only during the night, peripheral oscillators can be chemically phase shifted throughout the day. For example, injection of dexamethasone, a glucocorticoid receptor agonist, resets the phase of circadian liver gene expression during the entire 24-h day. Given the bewildering array of agents capable of influencing peripheral clocks, the identification of physiologically relevant agents used by the SCN to synchronize peripheral clocks will clearly be an arduous undertaking. Nevertheless, we feel that experimental systems by which this enticing problem can be tackled are now at hand.     

Photic resetting of the human circadian pacemaker in the absence of conscious vision

Ocular light exposure patterns are the primary stimuli for entraining the human circadian system to the local 24-h day. Many totally blind persons cannot use these stimuli and, therefore, have circadian rhythms that are not entrained. However, a few otherwise totally blind persons retain the ability to suppress plasma melatonin concentrations after ocular light exposure, probably using a neural pathway that includes the site of the human circadian pacemaker, suggesting that light information is reaching this site. To test definitively whether ocular light exposure could affect the circadian pacemaker of some blind persons and whether melatonin suppression in response to bright light correlates with light-induced phase shifts of thecircadian system, the authorsperformed experiments with 5 totally blind volunteers using a protocol known to induce phase shifts of the circadian pacemaker in sighted individuals. In the 2 blind individuals who maintained light-induced melatonin suppression, the circadian system was shifted by appropriately timed bright-light stimuli. These data demonstrate that light can affect the circadian pacemaker of some totally blind individuals--either by altering the phase of the circadian pacemaker or by affecting its amplitude. They are consistent with data from animal studies demonstrating that there are different neural pathways and retinal cells that relay photic information to the brain: one for conscious light perception and the other for non-image-forming functions.     

Circadian pacemaker neurons transmit and modulate visual information to control a rapid behavioral response

Circadian pacemaker neurons contain a molecular clock that oscillates with a period of approximately 24 hr, controlling circadian rhythms of behavior. Pacemaker neurons respond to visual system inputs for clock resetting, but, unlike other neurons, have not been reported to transmit rapid signals to their targets. Here we show that pacemaker neurons are required to mediate a rapid behavior. The Drosophila larval visual system, Bolwig's organ (BO), projects to larval pacemaker neurons to entrain their clock. BO also mediates larval photophobic behavior. We found that ablation or electrical silencing of larval pacemaker neurons abolished light avoidance. Thus, circadian pacemaker neurons receive input from BO not only to reset the clock but also to transmit rapid photophobic signals. Furthermore, as clock gene mutations also affect photophobicity, the pacemaker neurons modulate the sensitivity of larvae to light, generating a circadian rhythm in visual sensitivity.     

Circadian effects of light no brighter than moonlight

In mammals, light entrains endogenous circadian pacemakers by inducing daily phase shifts via a photoreceptor mechanism recently discovered in retinal ganglion cells. Light that is comparable in intensity to moonlight is generally ineffective at inducing phase shifts or suppressing melatonin secretion, which has prompted the view that circadian photic sensitivity has been titrated so that the central pacemaker is unaffected by natural nighttime illumination. However, the authors have shown in several different entrainment paradigms that completely dark nights are not functionally equivalent to dimly lit nights, even when nighttime illumination is below putative thresholds for the circadian visual system. The present studies extend these findings. Dim illumination is shown here to be neither a strong zeitgeber, consistent with published fluence response curves, nor a potentiator of other zeitgebers. Nevertheless, dim light markedly alters the behavior of the free-running circadian pacemaker. Syrian hamsters were released from entrained conditions into constant darkness or dim narrowband green illumination (~0.01 lx, 1.3 x 10(-9) W/cm(2), peak lambda = 560 nm). Relative to complete darkness, constant dim light lengthened the period by ~0.3 h and altered the waveform of circadian rhythmicity. Among animals transferred from long day lengths (14 L:10 D) into constant conditions, dim illumination increased the duration of the active phase (alpha) by ~3 h relative to complete darkness. Short day entrainment (8 L:16 D) produced initially long alpha that increased further under constant dim light but decreased under complete darkness. In contrast, dim light pulses 2 h or longer produced effects on circadian phase and melatonin secretion that were small in magnitude. Furthermore, the amplitude of phase resetting to bright light and nonphotic stimuli was similar against dimly lit and dark backgrounds, indicating that the former does not directly amplify circadian inputs. Dim illumination markedly alters circadian waveform through effects on alpha, suggesting that dim light influences the coupling between oscillators theorized to program the beginning and end of subjective night. Physiological mechanisms responsible for conveying dim light stimuli to the pacemaker and implications for chronotherapeutics warrant further study.     

Emergence of circadian and photoperiodic system level properties from interactions among pacemaker cells

Daily patterns of behavior and physiology in animals in temperate zones often differ substantially between summer and winter. In mammals, this may be a direct consequence of seasonal changes of activity of the suprachiasmatic nucleus (SCN). The purpose of this study was to understand such variation on the basis of the interaction between pacemaker neurons. Computer simulation demonstrates that mutual electrical activation between pacemaker cells in the SCN, in combination with cellular electrical activation by light, is sufficient to explain a variety of circadian phenomena including seasonal changes. These phenomena are: self-excitation, that is, spontaneous development of circadian rhythmicity in the absence of a light-dark cycle; persistent rhythmicity in constant darkness, and loss of circadian rhythmicity in pacemaker output in constant light; entrainment to light-dark cycles; aftereffects of zeitgeber cycles with different periods; adjustment of the circadian patterns to day length; generation of realistic phase response curves to light pulses; and relative independence from day-to-day variation in light intensity. In the model, subsets of cells turn out to be active at specific times of day. This is of functional importance for the exploitation of the SCN to tune specific behavior to specific times of day. Thus, a network of on-off oscillators provides a simple and plausible construct that behaves as a clock with readout for time of day and simultaneously as a clock for all seasons.     

Light- and temperature-entrainable circadian clock in soybean development

In plants, the spatiotemporal expression of circadian oscillators provides adaptive advantages in diverse species. However, the molecular basis of circadian clock in soybean is not known. In this study, we used soybean hairy roots expression system to monitor endogenous circadian rhythms and the sensitivity of circadian clock to environmental stimuli. We discovered in experiments with constant light and temperature conditions that the promoters of clock genes GmLCLb2 and GmPRR9b1 drive a self-sustained, robust oscillation of about 24-h in soybean hairy roots. Moreover, we demonstrate that circadian clock is entrainable by ambient light/dark or temperature cycles. Specifically, we show that light and cold temperature pulses can induce phase shifts of circadian rhythm, and we found that the magnitude and direction of phase responses depends on the specific time of these two zeitgeber stimuli. We obtained a quadruple mutant lacking the soybean gene GmLCLa1, LCLa2, LCLb1, and LCLb2 using CRISPR, and found that loss-of-function of these four GmLCL orthologs leads to an extreme short-period circadian rhythm and late-flowering phenotype in transgenic soybean. Our study establishes that the morning-phased GmLCLs genes act constitutively to maintain circadian rhythmicity and demonstrates that their absence delays the transition from vegetative growth to reproductive development.     

Melatonin rhythm observed throughout a three-cycle bright-light stimulus designed to reset the human circadian pacemaker.

Exposure to light and darkness can rapidly induce phase shifts of the human circadian pacemaker. A type 0 phase response curve (PRC) to light that has been reported for humans was based on circadian phase data collected from constant routines performed before and after a three-cycle light stimulus, but resetting data observed throughout the entire resetting protocol have not been previously reported. Pineal melatonin secretion is governed by the hypothalamic circadian pacemaker via a well-defined neural pathway and is reportedly less subject to the masking effects of sleep and activity than body temperature. The authors reasoned that observation of the melatonin rhythm throughout the three-cycle light resetting trials could provide daily phase-resetting information, allowing a dynamic view of the resetting response of the circadian pacemaker to light. Subjects (n = 12) living in otherwise dim light (approximately 10-15 lux) were exposed to a noncritical stimulus of three cycles of bright light (approximately 9500 lux for 5 h per day) timed to phase advance or phase delay the human circadian pacemaker; control subjects (n = 11) were scheduled to the same protocols but exposed to three 5-h darkness cycles instead of light. Subjects underwent initial and final constant routine phase assessments; hourly melatonin samples and body temperature data were collected throughout the protocol. Average daily phase shifts of 1 to 3 h were observed in 11 of 12 subjects receiving the bright light, supporting predictions obtained using Kronauer's phase-amplitude model of the resetting response of the human circadian pacemaker. The melatonin rhythm in the 12th subject progressively attenuated in amplitude throughout the resetting trial, becoming undetectable for >32 hours preceding an abrupt reappearance of the rhythm at a shifted phase with a recovered amplitude. The data from control subjects who remained in dim lighting and darkness delayed on average -0.2 h per day, consistent with the daily delay expected due to the longer than 24-h intrinsic period of the human circadian pacemaker. Both temperature and melatonin rhythms shifted by equivalent amounts in both bright light-treated and control subjects (R = 0.968; p<0.0001; n = 23). Observation of the melatonin rhythm throughout a three-cycle resetting trial has provided a dynamic view of the daily phase-resetting response of the human circadian pacemaker. Taken together with the observation of strong type 0 resetting in humans in response to the same three-cycle stimulus applied at a critical phase, these data confirm the importance of considering both phase and amplitude when describing the resetting of the human circadian pacemaker by light.     

Conditioning in the Orcadian System

Light is the dominant environmental cue for entrainment of circadian rhythms. In mammals, light entrains rhythms by resetting the phase of a circadian pacemaker located in the hypothalamic suprachiasmatic nucleus (SCN). Until recently, the mechanism responsible for pacemaker resetting by light was thought to be exclusively sensitive to photic cues. New experiments indicate, however, that this mechanism is more plastic than once thought; is amenable to conditioned stimulus control; and is capable of acquiring, through conditioning, new response capabilities. These experiments showed that, in rats, a neutral stimulus paired with light in Pavlovian conditioning trials is capable of eliciting cellular and behavioral effects characteristic of circadian clock phase resetting by light, expression of Fos protein in the ventrolateral region of the SCN, and phase shifts of free-running rhythms. These novel results open up a previously unappreciated perspective on photic phase resetting and entrainment of circadian rhythms. Specifically, they suggest that the process normally initiated by light to reset the clock can be modified by learning and events in the environment that reliably precede the onset of light can assume the resetting function of light.     

Involvement of the period gene in developmental time-memory: effect of the perShort mutation on phase shifts induced by light pulses delivered to Drosophila larvae

Phases of circadian locomotor activity rhythms of adult Drosophila reared in constant darkness have been shown to be set by a light stimulus delivered as early as the first-instar larval stage. This implies that a circadian clock functions continuously throughout postembryonic development. The clock genes period (per) and timeless (tim) are expressed cyclically in the larval central nervous system of Drosophila, and daily oscillations of per expression persist throughout metamorphosis in a group of cells, which gives rise to the pacemaker cells underlying locomotor activity rhythms of adults. Therefore, PER and TIM cyclings in these neurons may be responsible for the phenomenon of "larval time-memory." In the absence of any evidence for the involvement of these genes in such a developmental clock, and because circadian-pacemaker functions are underanalyzed in terms of the functions during development, the authors tested the time-memory of a fast-clock period mutant. They show that dark-reared perS mutant individuals as well as wild-type flies can be entrained as larvae and that a brief light pulse given to such entrained larvae can induce phase shifts in animals of either genotype. However, the direction and magnitude of phase shifts were different between wild type and perS, suggesting that a clock under the control of period gene participates in the regulation of developmental time-memory. The authors show that the relevant clock can be entrained by two light input pathways, one involving the phospholipase C encoded by the norpA gene, the other mediated by the blue-light receptor cryptochrome. Phase shifts of molecular oscillations during the larval stage were smaller than those measured by adult behavior, suggesting molecularly transient responses during development.     

Nonphotic entrainment in humans?

Although light is accepted as the dominant zeitgeber for entrainment of the human circadian system, there is evidence that nonphotic stimuli may play a role. This review critically assesses the current evidence in support of nonphotic entrainment in humans. Studies involving manipulations of sleep-wake schedules, exercise, mealtimes, and social stimuli are re-examined, bearing in mind the fact that the human circadian clock is sensitive to very dim light and has a free-running period very close to 24 h. Because of light confounds, the study of totally blind subjects with free-running circadian rhythms represents the ideal model to investigate the effects of nonphotic stimuli on circadian phase and period. Strong support for nonphotic entrainment in humans has already come from the study of a few blind subjects with entrained circadian rhythms. However, in these studies the nonphotic stimulus(i) responsible was not identified. The effect of appropriately timed exercise or exogenous melatonin represents the best proof to date of an effect of nonphotic stimuli on human circadian timing. Phase-response curves for both exercise and melatonin have been constructed. Given the powerful effect of feeding as a circadian zeitgeber in various nonhuman species, studies of meal timing are recommended. In conclusion, the available evidence indicates that it remains worthwhile to continue to study nonphotic effects on human circadian timing to identify treatment strategies for shift workers and transmeridian travelers as well as for the blind and possibly the elderly.     

A forced desynchrony study of circadian pacemaker characteristics in seasonal affective disorder

The circadian pacemaker is an endogenous clock that regulates oscillations in most physiological and psychological processes with a near 24-h period. In many species, this pacemaker triggers seasonal changes in behavior. The seasonality of symptoms and the efficacy of light therapy suggest involvement of the circadian pacemaker in seasonal affective disorder (SAD), winter type. In this study, circadian pacemaker characteristics of SAD patients were compared with those of controls. Seven SAD patients and matched controls were subjected to a 120-h forced desynchrony protocol, in which core body temperature and melatonin secretion profiles were measured for the characterization of circadian pacemaker parameters. During this protocol, which enables the study of unmasked circadian pacemaker characteristics, subjects were exposed to six 20-h days in time isolation. Patients participated twice in winter (while depressed and while remitted after light therapy) and once in summer. Controls participated once in winter and once in summer. Between the SAD patients and controls, no significant differences were observed in the melatonin-derived period or in the phase of the endogenous circadian temperature rhythm. The amplitude of this rhythm was significantly smaller in depressed and remitted SAD patients than in controls. No abnormalities of the circadian pacemaker were observed in SAD patients. A disturbance in thermoregulatory processes might explain the smaller circadian temperature amplitude in SAD patients during winter.     

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.     

The brain's calendar: neural mechanisms of seasonal timing

The suprachiasmatic nucleus (SCN) of the hypothalamus is the principal component of the mammalian biological clock, the neural timing system that generates and coordinates a broad spectrum of physiological, endocrine and behavioural circadian rhythms. The pacemaker of the SCN oscillates with a near 24 h period and is entrained to the diurnal light-dark cycle. Consistent with its role in circadian timing, investigations in rodents and non-human primates furthermore suggest that the SCN is the locus of the brain's endogenous calendar, enabling organisms to anticipate seasonal environmental changes. The present review focuses on the neuronal organization and dynamic properties of the biological clock and the means by which it is synchronized with the environmental lighting conditions. It is shown that the functional activity of the biological clock is entrained to the seasonal photic cycle and that photoperiod (day length) may act as an effective zeitgeber. Furthermore, new insights are presented, based on electrophysiological and molecular studies, that the mammalian circadian timing system consists of coupled oscillators and that the clock genes of these oscillators may also function as calendar genes. In summary, there are now strong indications that the neuronal changes and adaptations in mammals that occur in response to a seasonally changing environment are driven by an endogenous circadian clock located in the SCN, and that this neural calendar is reset by the seasonal fluctuations in photoperiod.