ABSTRACT
Sleep homeostasis refers to a regulatory process that counteracts transitory deviations of sleep from an average “reference level” (1). In the two-process model, a homeostatic (process S) and a circadian process (C) determine timing, duration and structure of sleep and wakefulness (2). Homeostatic process S and circadian process C interact to allow consolidated periods of sleep and wakefulness: the gradual increase of process S during waking is counterbalanced by a declining trend of endogenous sleep propensity, and the inverse relationship exists during sleep (Fig. 1).
