Since the rat is a nocturnal animal sleep experiments in this species are commonly performed during the light period of the 24-h light-dark (LD) cycle. To examine whether light itself affects sleep, chronically implanted albino rats were continuously recorded for a day under a 12-h light-12-h dark cycle (light intensity approx. 300 lx) and on the subsequent day in constant darkness (DD). In the absence of light, EEG slow-wave activity (0.75-4.0 Hz) in non-REM sleep and sleep continuity were significantly enhanced, while total sleep time and cortical temperature were not affected. The results show that in the albino rat room light impairs sleep by reducing its intensity and continuity.