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The purpose of this study is to examine aspects of operant behavior-modeled economic choice for food in rats in closed economy protocols in which food is available for only a few discrete times per daily 23-h session, designed to emulate clustering of human food intake into meals. In the first experiment, rats performed lever press responses for food pellets in an ascending series of ratios or fixed unit prices (FUP) when food was available for four 40-min food opportunities (FO) per day. Daily intake at low FUP was comparable to ad libitum intakes. Intake declined as FUP increased and was not distributed equally among the four FOs. In particular, the last FO of a session (occurring at about lights on in a 12:12cycle) was the smallest, even when total intake was low due to the response requirement at high FUP. Within FOs, satiation was evident at low FUPs by a decrease in rate of intake across a 40min FO; at high FUPs responding was evenly distributed. In the second experiment, rats had a choice of responding on two levers for either intermittent inexpensive (II; low FUP according to a four FO schedule) or costly continuous (CC; 20-fold higher FUP but available throughout 23-h sessions) food. Most (73%) of the rats consistently chose almost all of their food from the II source. Further, as the timing of the four II FOs were changed relative to the light: dark Zeitgeber, the time of the smallest meal changed such that the smallest meal (s) were during the light period regardless of ordinal position within a session. These data are discussed in terms of economic and Zeitgeber effects on consumption when food is available intermittently, and are contrasted with results from comparable protocols in mice.

We have shown previously that mice given access to four discrete feeding opportunities (FOs) per day show a characteristic sequence of sizes across ordinal FOs. The purpose of the present experiments was to determine the relative contributions of external and internal factors on the sequencing of FO size. The external factors were the light:dark Zeitgeber and the cost of food, imposed via different fixed unit prices (FUP) in a closed operant economy, and the internal factors were signals relating to energy status including time since last food and weight loss. In the first experiment, mice were given 4 FOs spaced 4-h apart, but with the timing of the FOs relative to the Zeitgeber altered by a 4-h Zeitgeber advance or delay of the cycle. Food intake, and associated body weight, declined as price increased, but the temporal order of FO size was invariant within a Zeitgeber condition. The Zeitgeber advanced group showed clear evidence of a shift in meal sequence relating to the light:dark cycle. Thus, external factors seem to be a more important determinant of total intake and sequencing than internal factors. In the second experiment, mice were given the choice between continuous costly (CC) and intermittent inexpensive (II) food. II food was available for four-15min intervals every 4-h, and the timing of the 15min intervals was varied relative to the Zeitgeber cycle. In spite of a 20-fold difference in price between CC and II food, mice took approximately equal amounts from each, and all food intake took place during the dark phase. Mice consumed II food only if it was available during the dark phase. Food intake was strongly linked to the light:dark cycle, largely independent of food cost.

The circadian system plays an important role in the temporal regulation of metabolic processes as well as food intake to ensure energy efficiency. The 'master' clock is located within the superchiasmatic nucleus and receives input from the retina so that it can be entrained by the light:dark cycle. In turn, the master clock entrains other clocks in the central nervous system, including areas involved in energy homeostasis such as the arcuate nucleus, and the periphery (e.g. adipose tissue and the gastrointestinal tract). This master clock is reinforced by other zeitgebers such as the timing of food intake and activity. If these zeitgebers desynchronise, such as occurs in high fat diet-induced obesity or shift work conditions, it can lead to a misalignment of circadian clocks, disruption of metabolic processes and the development of metabolic disorders. The timing of food intake is a strong zeitgeber, particularly in the gastrointestinal tract, and therefore time restricted feeding offers potential for the treatment of diet and shift work induced metabolic disorders. This review will focus on the role of the circadian system in food intake regulation and the effect of environment factors, such as high fat diet feeding or shift work, on the temporal regulation of food intake along with the benefits of time restricted feeding.

Serotonin (5-HT) is an important regulator of the mammalian circadian system, and has been implicated in modulating entrained and free-running rhythms, as well as photic and non-photic phase shifting. In general, 5-HT appears to oppose the actions of light on the circadian system of nocturnal rodents. As well, 5-HT mediates, at least in part, some non-photic responses. The 5-HT1A, 1B and 7 receptors regulate these acute responses to zeitgebers. 5-HT also regulates some entrained and free-running properties of the circadian clock. The receptors that contribute to these phenomena have not been fully examined. Here, we use 5-HT1A receptor knockout (KO) mice to examine the response of the mouse circadian system to a variety of lighting conditions, including a normal light-dark cycle (LD), T-cycles, phase advanced LD cycles, constant darkness (DD), constant light (LL) and a 6 hour dark pulse starting at CT5. Relative to wildtype mice, the 5-HT1A receptor KO mice have lower levels of activity during the first 8h of the night/subjective night in LD and LL, later activity onsets on transient days during re-entrainment, shorter free-running periods in LL when housed with wheels, and smaller phase shifts to dark pulses. No differences were noted in activity levels during DD, alpha under any light condition, free-running period in DD, or phase angle of entrainment in LD. While the 5-HT1A receptor plays an important role in regulating photic and non-photic phase shifting, its contribution to entrained and free-running properties of the circadian clock is relatively minor.

The drug of abuse methamphetamine (METH) is known for its ability to enhance reward responses. The rewarding properties of psychostimulants have been shown to vary across time of day in mice. The goal of this study was to determine the role of the MT1 and MT2 melatonin receptors in METH-induced reward, as measured by the conditioned place preference (CPP) paradigm during the light and dark phases. C3H/HeN wild-type mice were trained for METH-induced CPP at either ZT 6–8 (ZT: Zeitgeber time; ZT 0 = lights on), when endogenous melatonin levels are low, or ZT 19–21, when melatonin levels are high. These time points also correspond to the high and low points for expression of the circadian gene Period1, respectively. The locomotor response to METH (1.2 mg/kg, ip) treatment was of similar magnitude at both times, however only C3H/HeN mice conditioned to METH at ZT 6–8 developed a place preference. C3H/HeN mice with a genetic deletion of either the MT1 (MT1KO) or MT2 (MT2KO) receptor tested at ZT 6–8 or ZT 19–21 did not develop a place preference for METH, though both showed a similar increase in locomotor activity following METH treatment when compared to wild-type mice. We conclude that in our mouse model METH-induced conditioned place preference is dependent on time of day and the presence of the MT1 or MT2 receptors, suggesting a role for melatonin in METH-induced reward.

Circadian rhythms enhance survival and reproductive fitness of animals by promoting optimal timing of behavior and physiology with reference to geophysical changes in environment. Although light is considered the dominant stimulus for entraining circadian rhythms, social stimuli can also act as zeitgebers. The aim of this study was to analyze how Desert hamsters (Phodopus roborovskii) coordinate their behavior in time with that of animals of another competing species (Mongolian gerbils, Meriones unguiculatus). First, the behavior of hamsters was analyzed during a step-wise avoidance test. Two effects were observed: a) spatial separation if it was possible or b) shortening of the activity period due to contact without chance for avoidance. The latter finding was now further analyzed using a phase response curve (PRC). Here, phase shifts of Desert hamsters caused by single social interactions with Mongolian gerbils were quantified. Phase advances during the rest period were found at CT3 and CT9, a similar tendency was observed at CT6. A second phase advance was determined at CT18, coinciding with the end of the activity period. Then, it was tested whether additional activity during the stimulus was a trigger for the phase response. Although an increase in activity occurred especially when stimuli were applied during the rest period, there was no general relation between additional activity measured and the phase response shown. Overall, relevance of interspecific contact as nonphotic zeitgeber was indicated by phase shifts in a phase response curve. The shape of it can be explained by two behavioral adaptations; stress and contact avoidance.

In many environments, photoperiod is a reliable predictor of ecological conditions. Such predictability generally declines towards low latitudes as the yearly cycle becomes less marked. It has been claimed that photoperiodic effects are small in cavies and guinea pigs that reproduce throughout the year. We here investigated photoperiodic influences on the onset of puberty in female cavies (Cavia aperea) and show that photoperiod exerts a major influence. Female pups kept in groups of two matured at about 47 days when born into lengthening (from 10:14 to 12.5:11.5 L:D) and 79 days when born into shortening day length (from 145:9.5 to 12:12 L:D) and kept under identical short day conditions after weaning on day 20 of life (12.25:11.75 L:D). As shown previously, social conditions, especially the presence of an adult male, proved important modifiers of the onset of maturity in females. Differential stress cannot be responsible for the social effects on puberty as social conditions did not affect cortisol levels in young females. We conclude that photoperiod plays an important role in gearing the onset of cavy reproduction to the seasons and that specific male stimuli rather than unspecific effects of stressors accelerate female maturation. (C) 2008 Elsevier Inc. All rights reserved.

The Lesotho mole-rat is a social subterranean rodent that occurs at altitude in the Drakensberg mountain range. As a consequence of living permanently underground these animals rarely if ever are exposed to light. The visual system of African mole-rats is particularly regressed whereas the circadian system is proportionately conserved. This study investigated the locomotor activity patterns of 12 Lesotho mole-rats maintained under a range of different lighting regimes. The majority (91.7%) of mole-rats entrained their activity patterns to a LD photoperiod of 12L/12D. The mole-rats displayed a monophasic nocturnal activity preference. Under constant dark (DD) most of the mole-rats (83.3%) showed a free running circadian activity pattern with a tau of 23.8 h to 24.4 h (mean ± S.E.M.: 24.07 h ± 0.07 h; n = 10). The phase of the activity rhythms each mole-rat exerted during the previous LD-cycle did not change when the animals started free-running after being placed in constant conditions. The duration of re-entrainment to a second bout of LD 12:12 amounted to 9.4 ± 2.03 days (mean ± S.E.M., n = 10). Eleven mole-rats (91.7%) adjusted their locomotor activity rhythms to an inversed light regime DL 12:12 and displayed significant nocturnal activity preference. The animals required 9.73 ± 2.01 days (mean ± S.E.M., n = 11) to adjust to the DL-photoperiod. The Lesotho mole-rat thus possesses a functional circadian clock that responds to a photic zeitgeber.

Entrainment of circadian rhythms by zeitgebers is generally believed to conform to the principles of the so-called nonparametric theory of entrainment. Although seldom recognized, this theory presupposes that the response of the circadian system to photic stimulation is dependent on previous photic stimulation. The process of dark adaptation in the circadian system has been investigated previously in only two studies in golden hamsters. In this study, dark adaptation in the circadian system of the mouse was investigated through the measurement of phase shifts of the locomotor activity rhythm of animals maintained in constant darkness for varying intervals of time before receiving single light pulses. It was found that 3 weeks of darkness are necessary for attainment of full responsiveness to light, whereas even a single 1-h pulse is sufficient to suppress the responsiveness. Additional results of this study indicate that phase shifts of the locomotor activity rhythm are consistently associated with changes in free-running period in both mice and golden hamsters. Phase advances elicited by single light pulses are associated with a shortening of circadian period, whereas phase delays are associated with a lengthening of period. Each 1 h of phase shift is associated with a period change of approximately 3 min.

The effect of light-dark (LD) cycle and food availability was tested on the demand-feeding rhythm of single and groups of rainbow trout and European catfish. Under LD and free food access, most trout and catfish displayed, respectively, a diurnal and a nocturnal pattern of demand-feeding activity, whereas a few fish or groups of fish switched from diurnalism to nocturnalism or vice versa. In both species held under constant lighting conditions and a restricted feeding (RF) cycle (RF 20:4), the demand-feeding rhythm rapidly synchronised to food availability. The demand-feeding rhythm was under endogenous control and, in rainbow trout, periodogram analysis suggested the existence of two oscillators, one synchronised by photoperiod (LEO) and the other by food (FEO). When submitted to both LD and RF cycles, LD was, at least in the rainbow trout, the dominant zeitgeber synchronising the demand-feeding rhythm. In catfish, food availability rapidly synchronised demand-feeding rhythm. Finally, in both species, the synchronisation of single fish to LD or feed availability appeared slower than that of groups of fish, supporting the idea that social organisation affects the circadian activity in fish.

The adaptation of the locomotor activity rhythm to a daylight reversal was previously found to be faster in C57BL/6 mice, which present a low level of melatonin, than in C3H/He mice, which exhibit a large nocturnal melatonin peak. Because pinealectomy has been shown to accelerate resynchronisation time in rats after a daylight reversal, we investigated the involvement of melatonin in the resynchronisation rate of locomotor activity rhythm in C57BL/6 and C3H/He strains. We first tested the effects of melatonin, administered at zeitgeber time (ZT) 20 (with ZT0 corresponding to light onset) for the 3 days preceding the daylight reversal, on the reentrainment of locomotor activity rhythm in both strains. Second, the effects of S-22153, a melatonin receptor antagonist, on the reentrainment of locomotor activity rhythm in C3H/He mice were examined. S-22153 was administered for the 3 days preceding the daylight reversal either at ZT12 or at ZT20, i.e., when endogenous melatonin levels are respectively low and high. Melatonin significantly delayed the resynchronisation of locomotor activity rhythm in C57BL/6 mice without affecting this parameter in C3H/He mice. S-22153 significantly accelerated the resynchronisation in C3H/He mice when administered at ZT20, but had no effect when administered at ZT12. These results support the hypothesis that the differences between C3H/He and C57BL/6 in the reentrainment of their locomotor activity rhythm depend, at least in part, on the interstrain differences in melatonin synthesis.

Maternal entrainment of circadian activity rhythm was studied in the laboratory mouse (C57BL/6J). Pregnant mice gave birth in constant dim light and the mother raised the pups until Postnatal Day 18 (weaning). The wheel running activity of these pups was individually monitored from Day 18. It was found that the phases of pups' activity on the day of weaning were similar to the phases of their mother's rhythm (p < 0.001), indicating that maternal entrainment occurs in the C57BL/6J pups. When the mother mouse was cyclically presented to the pups for 12 h of a day, thereby creating presence-absence (PA) cycles of 12:12 h, it was found that the pups' activity rhythm entrained to the imposed cycles. The onset of activity of the pups coincided with the beginning of the mother's 12-h absence period. It is proposed that the social contact between the mother and the pups is taken by the pups as subjective day (rest time) and absence of the mother as subjective night (activity time). This maternal (nonphotic) entrainment, however, does not continue beyond Postnatal Days 23-26, despite ongoing PA cycles. These results indicate that the PA cycles of the mother are a transient zeitgeber, and are effective in entraining the rhythm of pups only for about 4 weeks after birth.

Rats with suprachiasmatic (SCN) lesions readily entrain to daily meals by increasing their activity prior to food access. Although previous experiments suggest that entrainment requires a nutritive meal, it is not yet clear what the parameters of the entraining stimulus are. The first experiment investigated the role of caloric content in resetting the feeding entrainable oscillator (FEO). Rats were entrained to 20 g chow/day until anticipatory activity was stable. The food access time was then delayed by 8 h and rats received either 0, 2, 6, or 16 g of chow for two days. Sixteen g of chow produced large delays on the next two cycles, while 0 and 2 g produced no delays. Two of 8 rats receiving 6 g showed delays, indicating that 22 kcal is near the threshold. In a second experiment, the effects of bulk were investigated. After an 8 h phase delay, rats received 0, 6, 10 or 16 g of chow mixed with cellulose for a total of 21 g for all groups. The 10 and 16 g chow groups delayed while the 0 g chow group did not. In the 6 g groups some animals phase shifted while others did not. Thus, the addition of non-nutritive bulk to the phase shifted meal had little or no effect on resetting the FEO and it appears that caloric content rather than gastric distention provides an effective zeitgeber for the FEO.

The effect of sound/silence cycles on the motor activity rhythm of the rat was studied. The daily motor activity pattern was studied in two groups of rats of both sexes kept first under DD and later under LL. One group (16 rats) was subjected to a cycle of sound and silence and the other (8 rats) was used as control. The study was performed under two different light regimes, to consider the possibility that a weak zeitgeber (such as sound might be), would exert an effect on the circadian pacemaker. This effect would depend on the stability of the circadian system, which could be modified by the lighting conditions. Results show no statistically significant differences between the two groups, neither in the motor activity patterns nor in the period of the rhythm. It was concluded that sound is not a zeitgeber for the motor activity rhythm of rats.

Sea bass, a fish species characterized by its dualistic feeding pattern, was investigated to study the synchronizing effect of light and food on the demand-feeding rhythm. Nocturnal and diurnal sea bass, both in groups and individually, were exposed to restricted-feeding (RF) and light-dark (LD) cycles of different periods. The phase relationship between both zeitgebers was also studied. The results show that food-demand rhythms synchronize to periodic food availability under constant light conditions (DD) and that there is a partial coupling between food-entrained and light-entrained activity under conflicting zeitgebers (LD 13:13 h and RF 4:20 h), suggesting the existence of a feeding entrainable oscillator (FEO) in addition to the master light entrainable oscillator (LEO). In some cases, food availability restricted to the light or dark phase contrary to that of the previous feeding phase changed a diurnal feeding pattern into nocturnal and viceversa, suggesting that food can be one of the switching factors that decides whether the circadian system of sea bass is nocturnal or diurnal. However, the fact that the feeding pattern of some fish was unrelated with the phase in which food was available suggests that other internal and/or external factors could be involved in the temporal flexibility of sea bass.

We have monitored parturition and different behavioral rhythms of does, the activity of suckling pups and behavioral rhythms of rabbits after weaning until adulthood. All animals were living in continuous light conditions (LL). As a consequence of the free-run of the does parturition, which takes place during the resting period of the animal, occurred at almost any time of the day. The once/day nursing was advanced during the first 7-10 days until it was in synchrony with the time of main activity. The pups significantly anticipated nursing by day 5: the 60 min bin prior to nursing exceeded the 24-h mean by 63 +/- 30%. The anticipatory activity (AA) was entrained by an artificial nursing schedule of T = 24 h, the 60 min bin prior to nursing exceeding the 24-h mean by 123 +/- 50%. Since AA persisted during a 48 h fast, its phase is controlled endogenously. At weaning behavioral rhythms of rabbits free-ran with a main tau24 h (average 23:35 +/- 17'), period length extending with increasing age. 7/10 animals had an additional component24 h (average: 24:24 +/- 16') which became increasingly dominant, the components24 h fading out within 30 days after weaning. Between days 340-350 a stable period length of 24:35 +/- 8' and a consolidation of the circadian rhythms was attained. These results confirm that during the first days of the rabbit's life the circadian system is controlled by a feeding entrainable oscillator, the light entrainable oscillators taking over control later.

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.

The free-running circadian rhythms of five behavioral functions of the rabbit were masked by unsignalled restricted food access (RF). The rhythms were reorganized immediately, a large part of events being assembled around the end of food availability. In addition to masking a slower process of entrainment was running: a component of anticipatory activity (AA) was established 1-3 h before food access. AA consolidated in continuation of the camouflaged free-running rhythm. The time of AA establishment correlated significantly with the phase angle difference (PAD) between free-running rhythm and RF: it decreased with decreasing positive PAD. With the consolidation of AA, the rhythms had attained a stable phase relation to RF. At the termination of RF, the circadian rhythms started to free-run again, the phase being dependent from that of the preceding RF schedule. The period length of the RF zeitgeber (TRF) had some impact on tau of the circadian rhythm. This aftereffect was most evident after termination of TRF less than 24 h: the free-running rhythm continued for up to 49 days with a tau less than 24 h and turned longer thereafter. The results demonstrate the entrainment of circadian rhythms of the rabbit by RF in addition to masking.

Variations of core-temperature rhythms occurring during a “normal” day (24-hour period without extraordinary challenges for organism) were studied in 5 male sheep. To record the influence of the metabolic processes in different organic systems, core temperatures were measured at various locations at the same time. To minimise any influences due to measurement or behavior (e.g., effects of isolation), a telemetric system was used for registration and animals were kept without restraint in their habitual herd. Particularly biphasic circadian rhythms were found, and feeding schedule as zeitgeber is discussed. Beyond that, independent from point of measurement short-time rhythms with wavelengths of 140 and 90 min were found. Therefore, an origin in the central nervous system can be supposed. Rhythms with wavelengths of 3 hours, 75 min and 1 hour were not stable throughout a whole 24-hour period and did not occur at all measurement points in the same intensity. A comparison of anatomical placements of the various measurement points leads to the realization of distinct organic functions as sources for these rhythms.

The interaction between light-entrainable rhythms (LER) and feeding-entrainable rhythms (FER) of wheel running was studied by maintaining 16 rats in constant darkness and on ad lib feeding for 14 days, followed by a restricted feeding schedule (2 hr food/day) for 42 days, and 25 days on ad lib feeding. In 4 rats the LER assumed the same period as food access during restricted feeding. The pre-restriction periods of the LER of these rats differed from that of the feeding schedule by 5 min or less. However, since food access did not determine the phase of the LER, food does not appear to be a true Zeitgeber for this rhythm. No synchronization was observed when the two periods differed by more than 10 min, although changes in period or phase of the LER during restricted feeding were observed in all rats. Four hour phase advances or delays of the feeding schedule did not result in consistent effects on the phase of the LER. Two hour light pulses near the onset or end of the LER resulted in phase delays and phase advances of the LER, but had no effect on the FER. In the context of a multioscillator circadian system, these results suggest that light- and feeding-entrainable circadian oscillators are only weakly coupled.

The effect of a cycle of warm and cool ambient temperature (Ta) upon the free-running circadian running and drinking rhythms of male and female laboratory rats was investigated. Rats free-running in constant darkness and constant cool Ta (21 degrees C +/- 2 degrees C) were exposed to a 12:12 cycle of high (34 degrees C +/- 2 degrees C) and cool (21 degrees C +/- 2 degrees C) Ta. Three male rats and one female rat entrained to the Ta cycle. Ten of 12 male and 9 of 11 female rats exhibited post-Ta cycle phases not predictable from pre-Ta cycle phases. Most rats exhibited positive and negative masking of activity during the Ta cycle. Activity periods shortened for all rats during the Ta cycle, and male free-running periods lengthened upon cessation of the Ta cycle to values significantly greater than precycle periods. It was concluded that Ta acts as a weak zeitgeber in laboratory rats and has greater effects on males compared to females.

Effective light intensities for entrainment of activity rhythms were obtained in the retinal degenerated C3Hf/He mice and the normal C57BL/6J mice. The circadian activity rhythms of the mice were examined under LD 12:12 cycles with one of five different levels of light (L phase) and complete darkness (D phase). The thresholds of a Zeitgeber, defined as the light intensity effective for 50% entrainment, were estimated between 1 lux and 5 lux in the C3H and lower than 0.01 lux in the C57BL mice. These results suggest that at least two different kinds of photoreceptor including rods and cones participate in the photic entrainment of the circadian activity rhythms.

To investigate the mechanism of circadian synchronization by oral food consumption, we tested the effectiveness of cyclic intragastric feeding in synchronizing squirrel monkeys to a 24-hour schedule. Five monkeys were prepared with intragastric catheters and were studied free-ranging within a cage in an environmentally controlled chamber with continuous monitoring of body temperature (every 15 min) and locomotor activity (every 30 min). A balanced mixture of dextrose, amino acids, fat emulsion, vitamins and minerals was infused from 0800-2000 every day via the gastrostomy, first during a 12-hour light and 12-hour dark cycle (lights on 0800-2000 hours), and then subsequently in constant illumination. Serial phase analysis was performed by computing cycle-by-cycle acrophases of best-fitting sinusoids. One animal was synchronized by the feeding regimen while four others exhibited relative coordination. We conclude from these results that the timing of intragastic feeding may have only a weak synchronizing effect on primate circadian rhythms and that pre-gastric cues associated with oral food ingestion contribute to entrainment by feeding schedules.

Spontaneous running activity was studied in individual female S1 rats under controlled conditions. Matched siblings, pinealectomized (p), sham-operated (s) and unoperated (u) at 2212–2412 weeks of age, were studied in their responses to four successive reversals (ΔΦ = 12) of the environmental photoperiod (LD 12:12; 200–800:3–15 lux) when they were 2712–3712 weeks old. Initial mean shift in activity starting time following ΔΦ was 6–7 hr ahead in the p animals as compared with the controls, and mean re-entrainment time, according to activity starting time, was 3.03 ± 1.00 days in the p animals as compared to the controls (s −8.60 ± 0.97; u −9.36 ± 1.16; p < 0.01). The post ΔΦ activity of the p animals also differed in showing a sharp differentiation of circadian (pre-lights-off) and photoperiod-dependent (post-lights-off) segments of activity phase (α), and in having an unusually close temporal relation and suggested dependency to the white-lights-off Zeitgeber during the first days following ΔΦ. Possible contributions of age, experience and central mechanisms underlying the pineal effects are discussed.