[Circadian regulation of the sleep-wake cycle].
A review of the studies considering the sleep/wake alternation as a biological circadian cycle is presented. The methods designed to evaluate respectively the role of both the endogenous component and the external synchronizers of isolation (free-running protocols). There are mutual relations between wake/sleep cycle and some other circadian rhythms, particularly body temperature rhythm. When sleep is subjected to any time manipulation, the homeostatic regulation of some sleep characteristics (REM-sleep amount, sleep onset probability, sleep length) remains very strong. Alertness appears to be linked both to sleep characteristics and to internal temperature evolution. In addition, it proved to present some degree of independence of the two afore mentioned factors, specially when motivation and/or other psychological factors are involved. The afternoon nap, far from being simply after lunch reaction, is a normal manifestation of circadian rhythmicity.