Sleep regulation in humans

The interest in the modeling approach to sleep regulation has increased over the past decade. Models help delineate the processes involved in the regulation of sleep and thereby offer a conceptual framework for the analysis of existing and new data. Sleep homeostasis denotes a basic principle of sleep regulation. A sleep deficit elicits a compensatory increase in the intensity and duration of sleep, while excessive sleep reduces sleep propensity. It is as though 'sleep pressure' is maintained within a range delimited by an upper and lower threshold. Sleep homeostasis is represented in the two-process model of sleep regulation by process S that increases during waking and declines during sleep. The timing and propensity of sleep are modulated also by a circadian process. Electroencephalographic (EEG) slow-wave activity (SWA) serves as an indicator of sleep homeostasis in non-rapid eye movement sleep. The level of SWA, a correlate of sleep intensity, is determined by the duration of prior sleep and waking. Evidence is accumulating for the existence of a local, use-dependent facet of sleep regulation.