Circadian regulation of sleep propensity, sleep structure and alertness: A symphony of paradoxes

The adult human typically exhibits a monophasic sleep-wake cycle, i.e., remains awake and alert for approximately 16 hours and then sleeps for 8 hours. Recent experiments have provided new insights in the role of the endogenous circadian pacemaker in this consolidation of sleep and wakefulness. Sleep deprivation studies had shown previously that sleepiness and alertness are co-determined by a process which keeps track of the history of sleep and wakefulness and the circadian pacemaker, which keeps track of time. During every day life and during sleep deprivation both processes change simultaneously and their relative contribution to alertness and sleep propensity cannot be assessed under these conditions.