ABSTRACT
Sleep and waking are well-defined behavioral and physiological states that in many animals, including humans, normally occur at restricted times of day. The timing of sleep and waking is regulated by two processes: (1) a circadian pace maker, which is entrained to the light-dark cycle and promotes wake during an active phase of the cycle and permits sleep during a rest phase of the cycle, and (2) a homeostatic process in which a need for sleep accumulates during waking and is dissipated during sleep (1-3). In addition, a third mechanism controls the ultradian alternation between the distinct states of sleep-rapid eye movement (REM) and non-REM sleep.
