Individual variability and photic entrainment of circadian rhythms in golden spiny mice

Golden spiny mice are diurnally active in most of their natural habitat. Their diurnal activity is ascribed to non-photic cues: competitive exclusion from the nocturnal niche, or thermoregulatory considerations. Here we studied the entrainment of golden spiny mice to light. In the laboratory, golden spiny mice were primarily nocturnal and displayed an unusual variety of rhythm patterns, with activity bursts occurring during both activity and rest periods. Spontaneous shifts of activity rhythms between light phases were sometimes recorded. In all cases but one, body temperature shifted in parallel with activity. Under DD conditions, the free running period (tau) of all individuals but one was shorter than 24 h, and in all individuals but the same one it was shorter than tau under LL conditions. In response to a 6 h phase delay, all individuals entrained to the new LD cycle in a relatively uniform way. During phase advance four out of the twelve individuals further delayed their activity and body temperature rhythms, and eight individuals advanced their activity rhythm, but the re-entrainment took them over twice as long as to re-entrain to the phase delay. We suggest that the golden spiny mouse is a nocturnal rodent whose circadian system developed the flexibility to be nocturnal or diurnal according to environmental conditions, or a nocturnal rodent in the process of turning diurnal, and that it has low sensitivity to the immediate masking effect of light on activity.

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.

Beyond the suprachiasmatic nucleus

The suprachiasmatic nucleus is the master oscillator controlling circadian rhythms in mammals. Yet extensive temporal restructuring of behavior can occur without participation of the suprachiasmatic nucleus. This raises questions about current thinking about how to cope with jet lag and shift work.