CIRCADIAN-RHYTHMS IN NORMAL AND DISORDERE D SLEEP

Different challenges, constraints for quality of sleep and wakefulness , sleep in extreme conditions, search for a better identification of d isorders of sleep and wakefulness, spur researches on the circadian rh ythm of sleep and on its disorders. Any advance in that field rests on the understanding of different physiologic phenomena: the intrinsic p eriod of our internal clock is not 24-hours but a little less than 25- hours and it must be reset daily by the Zeitgebers. A sudden phase shi ft provokes an external desynchronisation. In addition not all rhythms resynchronize together resulting in an internal desyncronization. The re is a close relationship between quantities of total sleep and REM s leep and the time of sleep onset within the 24-hours. The propensity t o sleep follows a circasemidian rhythm with a major peak during the ni ght and a secondary peak in the midafternoon. Disorders of the circadi an rhythm of sleep are of two types: related to the misalignment of th e sleep-wake schedule and the synchronizer-controlled rest-activity rh ythm (shift work sleep disorder and time zone change syndrome) or rela ted to an abnormal escape of the individual rest-activity rhythm from synchronizer control (delayed sleep phase syndrome, advanced sleep pha se syndrome and non 24-hour sleep-wake syndrome.