Hormones and Sleep

There are several features of the interaction between sleep and circadian rhythmicity which appear to be unique to the human species. First, human sleep is generally consolidated in a single 6–9-h period, whereas fragmentation of the sleep period in several bouts is the rule in other mammals. Possibly as a result of this consolidation of the sleep period, the wake-sleep and sleep-wake transitions in man are associated with physiological changes which are usually more marked than those observed in animals. For example, the secretion of growth hormone (GH) in normal adults is tightly associated with the beginning of the sleep period whereas the relationship between GH secretory pulses and sleep stages is much less evident in rodents, primates and dogs. Secondly, humans are also unique in their capacity to ignore circadian signals and to maintain wakefulness despite an increased pressure to go to sleep. Finally, human subjects maintained for prolonged periods of time in so-called temporal isolation, i.e., in natural or artificial environments devoid of any periodicity (e.g., light-dark cycle) or temporal cues (i.e., clocks or watches), may show behavioral modifications which have not been observed in laboratory animals under constant conditions.These modification consist of a desynchronization between the sleep-wake cycle and other rhythms, such as those of body temperature and cortisol secretion, which persist with a period of around 24h (Aschoff 1979;Wever 1979).