Sleep states and electroencephalographic (EEG) parameters were determined in unrestrained rats that had been implanted with electrodes under deep pentobarbital sodium anesthesia. Two base-line days with a light-dark cycle (LD) and 2 days under continuous darkness (DD) were followed by 24 h of sleep deprivation (SD) ending in the middle of the circadian activity period and by 2 recovery days in DD. In the base-line LD rest period, the amount of rapid-eye-movement sleep (REMS) and the EEG amplitude of non-REMS (NREMS) were lower than in the corresponding DD period. SD caused an immediate enhancement of REMS, NREMS, the slow-wave sleep (SWS) fraction of NREMS, and NREMS EEG amplitude. Although REMS, NREMS, and SWS showed a second peak at habitual light onset, they did not exceed base line. Subsequently, all parameters exhibited a marked negative rebound. We conclude that REMS and the EEG amplitude of NREMS are suppressed by light, amplitude and frequency parameters of NREMS are differently affected by light as well as by SD, and the short duration of the SD-induced increase of SWS may reflect a circadian influence on sleep homeostasis.