Vigilance states and EEG power density of the rat were determined after a 3- or 6-h sleep deprivation (SD) in the beginning of the 12-h light period. In comparison to baseline, non-rapid eye movement (REM) sleep showed a delayed and transitory increase after 3 h SD, and an immediate and persistent increase after 6 h SD. REM sleep was not affected. In non-REM sleep, EEG power density in the low-frequency range (0.75-6.0 Hz) was markedly enhanced after 6 h SD, but not significantly increased after 3 h SD. In REM sleep EEG activity in the 5-6 Hz band was increased after 6 h SD. We conclude that in the early part of the light period, 3 h waking prolongs non-REM sleep, whereas 6 h waking also enhances non-REM sleep intensity.