Processes Underlying the Regulation of the Sleep-Wake Cycle

Abstract

Sleep and waking are the two major functional states which in homeotherms can be unambiguously identified by both behavioral and electrophysiologic criteria. Sleep and waking are closely associated with rest and activity. The circadian rest-activity rhythm is present in organisms in which the usual criteria for sleep and waking cannot be applied (e.g., in invertebrates). In fact, sleep and waking may have evolved from the states of rest and activity whose propensity is modulated by the circadian pacemaker. It has been argued that the emergence of sleep has brought an increased flexibility by loosening the control of the circadian pacemaker over behavior and enabling a need-dependent recovery (Borbely and Neuhaus, 1979). It is in particular the intensity dimension of sleep which can be viewed as allowing sleep to fulfil its putative needdependent functions without disrupting the circadian sleep-wake rhythm. As will be shown, there is evidence that the circadian and sleep-wake-dependent aspects of sleep regulation are based on separate mechanisms. Three distinct processes can be identified which underlie the regulation of the sleepwake cycle in humans and other mammals (Figure 1) : (1) A sleep-wake dependent , homeostatic process determined by prior sleep and waking, (2) a clock-like cii-cadian process that is largely independent of sleep and waking, and (3) an ~iltradian process that is characterized by the alternation of the two basic sleep states, non-rapid-eye-movement (nonREM) sleep and REM sleep.