Temporal orientation: circadian clocks in animals and humans

By evolutionary adaptation to the regular day-night changes in environmental conditions, most organisms have acquired a temporal programme which matches the 24-h day. It rests on periodic processes which have the characteristics of self-sustaining oscillations, and which, in constant conditions, free-run with periods slightly deviating from 24 h. When entrained to 24 h, these circadian rhythms can be used by the organism as a clock for temporal orientation, e.g. in the occupation of one of the temporal niches provided by the environment or in the coordination of activities among individuals and species. The circadian clock also provides the basis for using the sun as a compass in spatial orientation, and for the recognition of the time of day as exemplified by the time sense in honey bees. Within the human organism, almost every function is modulated in a circadian fashion. Usually, all rhythms keep a distinct phase-relationship t to each other, providing a high degree of temporal order within the organism. When living in an isolation unit without time cues, most subjects develop free-running rhythms with periods close to 25 h in all functions. Sometimes, however, the sleep-wake cycle is lengthened to 30 h and more, or shortened to less than 22 h. In those instances, other rhythms such as that of body temperature become uncoupled from the sleep-wake cycle and continue to free-run with a period of about 25 h. During such states of ‘internal desynchronization’, subjects can be awake continuously for about 32 h, and sleep without interruption for 16 h. Nevertheless, they experience their ‘days’ as equal to 24 h. This is due to a profound change in time estimation: the intervals of 1-h estimates made by the subject are positively correlated with the duration of wakefulness. In contrast, short-time estimates (in the range of seconds) remain unaffected by desynchronization, indicating that short- and long-time estimates are based on different mechanisms.