Sleep states, the power spectra of the cortical electroencephalogram (EEG) and cortical temperature (Tcrt) were determined in young rats (age 23-24 days). Recordings were made for 1 day under habitual 12 h light: 12 h dark (LD 12:12) conditions and on the subsequent day under continuous darkness (DD). The amount and distribution of the vigilance states differed little between experimental conditions. Sleep occurred predominantly during the actual (LD) or habitual (DD) 12-h light period. The EEG power density in the actual light period was lower than in the habitual light period. These differences were largest in the delta range for the EEG of non-rapid eye movement of sleep (NREMS) and in the theta range for the EEG of REM sleep (REMS) and waking. EEG power density in NREMS was somewhat lower in the LD dark period than in the corresponding DD period. The typical 24-h pattern of EEG power density in NREMS, which reflects processes underlying sleep regulation, was little affected by the experimental conditions. It is concluded that the light during an LD 12:12 schedule suppresses the EEG but has little effect on the vigilance states.