The homeostatic regulation of non-rapid eye-movement sleep (NREMS) consists of the changes in both duration and intensity, the latter being reflected by EEG slow-wave activity (SWA). In some species, the seasonal changes have a prominent effect on behavior and on physiological variables, and specific, seasonally recurring behaviors, such as breeding and torpor, are triggered by the photoperiod. During emergence from torpor, SWA rapidly increased to a level that significantly exceeded the baseline value. The increase of SWA and the spectral changes of the sleep EEG after emergence from torpor closely resembled the increase of SWA after 4-hour sleep deprivation. However, the effect of torpor was shorter lasting. To control for the metabolic exertion during the rise of body temperature, the emergence from torpor was promoted by heating the animal. The increase of SWA after this manipulation did not differ from the rise following the unaided termination of torpor. Taken together, these studies, as well as the hibernation experiments, strongly indicate that Process S builds up not only during waking but also in the course of other non-sleep states. They point to the exquisite role of sleep that alone can reduce the level of S.