ABSTRACT
Sleep is a regulated process whose timing, duration, and intensity depend on the prior sleep-wake history (1). Sleep deprivation refers equally to the restriction of sleep or the extension of wakefulness beyond the habitual time span. Sleep propensity, i.e., the need or inclination to initiate and maintain sleep, becomes uncontrollable when sleep deprivation is extensively protracted, even under relaxed conditions. It is accepted that sleep has a restorative function that cannot be substituted simply by resting. Indeed, muscles and most organs can get the same amount of rest in relaxed wakefulness as in sleep [reviewed in (2)]. But for the brain, relaxed wakefulness is not sufficient to substitute for sleep, as revealed by the specific and dramatic effects of sleep deprivation on human neurobehavioral functions (3-7).
