Daily exercise promotes physical health as well as improvements in mental and neural functions. Studies in intact wild-type (WT) rodents have revealed that the brain’s suprachiasmatic nuclei (SCN), site of the main circadian pacemaker, are also responsive to scheduled wheel running. It is unclear, however, if and how animals with a dysfunctional circadian pacemaker respond to exercise. Here, we tested whether scheduled voluntary exercise (SVE) in a running wheel for 6 hours per day could promote neural and behavioral rhythmicity in animals whose circadian competence is compromised through genetically targeted loss of vasoactive intestinal polypeptide (VIP^-/- mice) or its VPAC₂ receptor (Vipr2^-/- mice). We report that in constant dark (DD), rhythmic VIP^-/- and Vipr2^-/- mice show weak free-running rhythms with a period of <23 hours and all wild-type mice are strongly rhythmic with approximately 23.5-hour periodicity. VIP^-/- and Vipr2^-/- mice rapidly (<7 days) synchronize to daily SVE, while WT mice take much longer (>35 days). Following 21 to 50 days of SVE, WT mice show small changes in their rhythms, and most Vipr2^-/- mice now sustain robust near 24-hour behavioral rhythms, whereas very few VIP^-/- mice do. This study demonstrates that scheduled daily exercise can markedly improve circadian rhythms in behavioral activity and raises the possibility that this noninvasive approach may be useful as an intervention in clinical etiologies in which there are dysfunctions of circadian time keeping.

Much is known about how environmental light-dark cycles synchronize circadian rhythms in animals. The ability of environmental cycles of ambient temperature to synchronize circadian rhythms has also been investigated extensively but mostly in ectotherms. In the present study, the synchronization of the circadian rhythm of running-wheel activity by environmental cycles of ambient temperature was studied in laboratory mice. Although all mice were successfully entrained by a light-dark cycle, only 60% to 80% of the mice were entrained by temperature cycles (24-32 °C or 24-12 °C), and attainment of stable entrainment seemed to take longer under temperature cycles than under a light-dark cycle. This suggests that ambient temperature cycles are weaker zeitgebers than light-dark cycles, which is consistent with the results of the few previous studies using mammalian species. Whereas 80% of the mice were entrained by 24-h temperature cycles, only 60% were entrained by 23-h cycles, and none was entrained by 25-h cycles. The results did not clarify whether entrainment by temperature cycles is caused directly by temperature or indirectly through a temperature effect on locomotor activity, but it is clear that the rhythm of running-wheel activity in mice can be entrained by ambient temperature cycles in the nonnoxious range.

Circadian clocks enable organisms to anticipate changes of environmental conditions. In social insects, the colony as a superorganism has a foraging rhythm aligned to the diurnal patterns of resource availability. Within this colony rhythm, the diurnal patterns of individuals are embedded, and various tasks within the colony are performed at different times by different individuals to best serve the colony as a whole. Recent studies have shown that social cues influence the traits of the circadian clock in social insects, but keeping track of the activity of individual workers is not an easy task. Here the authors use fully automatic radio-frequency identification (RFID) to analyze the circadian rhythms of bumblebee foragers (Bombus terrestris) in the normal social context of their nest. They monitored their foraging patterns under different light conditions in the laboratory, including light:dark cycles (LD) as well as constant darkness (DD) and constant light conditions (LL). Their results show that the majority of bumblebee foragers exhibit robust circadian rhythms in LD under laboratory conditions, while they show free-running rhythms both in DD and LL, with free-running periods being significantly shorter in LL conditions. The authors also found that bumblebee workers show an increased level of arrhythmic activity ("death dance") in the hours or days before their death.

The Northern Wheatear (Oenanthe oenanthe) is a migratory bird species that shows different strategies of migration between populations, adapted to cope with different ecological barriers. This raises the question whether and to which extent these adaptations are endogenously determined. We studied seasonal patterns of body mass change and nocturnal restlessness in wheatears from Iceland, which face an initial sea crossing of at least 800 km; from Norway, which fly a similar distance as Icelandic birds but without a long sea crossing; and from Morocco, which fly a shorter distance to reach their wintering grounds. To isolate the endogenous component of the regulation of these migratory traits, we kept the wheatears in a "common garden," all 3 populations experiencing the same environmental conditions and a constant photoperiod during their first year of life. Icelandic birds showed a greater increase of their body mass in autumn than the other 2 populations, indicating preparation for the initial barrier crossing. The autumnal timing of nocturnal restlessness and the total activity during autumn were related to the distance to be covered, although the differences between populations were smaller than expected. In all 3 populations, body mass increased to a greater extent in autumn than in spring, whereas nocturnal activity was higher in spring than in autumn. This suggests that the endogenous program responds to specific seasonal needs, with more time invested in storing fuel for a safe journey in autumn and more time invested in flying to reach the breeding grounds early in spring. Contrary to expectations, the timing of onset of body mass increase and nocturnal restlessness in spring did not differ between populations. This might be explained by the lack of external cues, most likely photoperiod, which are responsible for the fine tuning of the expression of migratory behavior.

The temporal distribution of behavioral programs throughout the 24-h day, known as temporal niche of a species, is determined by ecological factors that directly affect the adaptive value of the timing of specific behaviors. Temporal niche switching has been described in several species and is likely adaptive in habitats where the daily timing of those factors changes. Benthic species whose habitats span a wide range of water depths are exposed to considerable depth-dependent environmental changes. Temporally scheduled trawl surveys of the Norway lobster, Nephrops norvegicus, reveal that animals emerge from burrows at night on the shallow shelf (10-50 m deep), at crepuscular hours on the lower shelf (50-200 m), and at daytime on the slope (200-400 m). The mechanisms underlying nocturnality/diurnality switches are chiefly unknown, and Nephrops offers a unique model for their study. The depth-dependent decrease in luminance is a likely candidate determining the temporal distribution of behavior. The authors explored this possibility in the laboratory by exposing Nephrops to light:dark (LD) cycles of 470-nm monochromatic lighting that mimic conditions at the 100-m-deep shelf (10 lux) or the 300-m slope (0.1 lux). Two groups of animals were respectively exposed to each light intensity according to the following protocol: an initial 12:12 LD stage followed by constant darkness (DD), followed in turn by a second 12:12 LD stage. Activity at the burrow opening (door-keeping = DK), as well as full emergence (E), was continuously monitored. Under 10-lux LD cycles, most animals showed nocturnal DK activity—with some being crepuscular or diurnal—and all animals showed nocturnal E activity. In contrast, both behaviors were clearly diurnal in animals under 0.1-lux LD cycles. The phase of the nocturnal and diurnal DK rhythms detected respectively at 10 and 0.1 lux upon release into DD revealed that these rhythms are entrained circadian rhythms. The present data indicate that nocturnality/diurnality switches in Nephrops in its natural habitat, evidenced by captures at different depths, are likely determined by light intensity. This temporal niche switching involves different patterns of photic entrainment, leading to bona fide circadian diurnal or nocturnal phenotypes, as well as exogenous masking of behavioral outputs.

Studies of sex differences in the timing of human circadian rhythms have reported conflicting results. This may be because the studies conducted to date have not controlled for the masking effects of the rest activity cycle on the circadian rhythms being assessed. In the present analysis of data collected under controlled conditions, we examined sex differences in the timing of circadian rhythms while minimizing masking from behavioral and environmental factors using a constant routine (CR) protocol. All participants (28 women and 28 men paired by habitual wake time; age range, 18 30 years) maintained a regular self selected sleep wake schedule at home prior to the study. After 3 baseline days in the laboratory, participants began a CR. Women were found to have a signifi cantly higher melatonin amplitude and lower temperature amplitude than men. While sleep timing was the same between the 2 groups, the timing of the circa dian rhythms of core body temperature and pineal melatonin secretion was ear lier relative to sleep time in women as compared to men. Sleep therefore occurred at a later biological time for women than men, despite being at the same clock time. Given that sleep propensity and structure vary with circadian phase and are impacted by circulating melatonin, these findings may have important impli cations for understanding sex differences in sleep timing and duration, diurnal preference, and the prevalence of sleep disorders such as insomnia.

A naturalistic pattern of dawn simulation (93 min, 0.001-250 lux) delivered to sleeping patients with winter depression has clinical efficacy similar to postawakening bright light therapy. The authors tested the circadian phase-shifting capacity of the dawn signal in a placebo-controlled, randomized 3-week field trial using 4 photic or nonphotic comparators: 30-min, 10,000-lux bright light therapy; a 13-min, 250-lux light pulse at the end of sleep; and high- and low-density negative air ionization timed to match the dawn. Bright light and dawn treatments both produced average phase advances of ~30 min, while the pulse and ion exposure had minimal effect. The authors conclude that very dim, incremental levels of light at the end of the night, with a dominant red component through eyelid filtering, facilitate circadian rhythm phase advances, possibly in conjunction with room light after awakening.