In the guinea pig, sleep and slow-wave activity (SWA) are evenly distributed over 24 h in contrast to the sleep pattern in other rodents where a daily preference for sleep and a decline of SWA within the main sleep period is typical. SWA is regulated as a function of prior waking and is assumed to reflect sleep intensity. We investigated sleep homeostasis and its possible circadian modulation by performing sleep deprivation (SD) at two phases of the light-dark cycle. SD induced an increase in SWA which was similar under both conditions. We conclude that sleep is homeostatically regulated in the guinea pig and is not subject to a circadian modulation.