This study was designed to examine the differences between sleep duration and EEG when sleep was restricted to the rest- or activity-phase for 5 successive days, achieved by repeated sleep deprivation in the dark (DSD) or light-period (LSD). In the DSD-experiment the percentages of the vigilance states were comparable to the level of the baseline light period. In LSD, the amounts of all sleep states increased substantially relative to baseline dark. The sleep episodes were lengthened in DSD and LSD. The duration of NREM-sleep and the sleep episodes remained longer in the light than in the dark, indicating circadian influences. In the first hours after sleep deprivation the delta activity during NREM-sleep was enhanced in LSD and to a lesser extent in DSD. This effect diminished over the consecutive days in both experiments. The EEG energy gained during sleep and its accumulation pattern on each day in DSD and LSD were strikingly similar, thereby reflecting a homeostatic process. After the sleep deprivation days, small changes were observed in the distribution of the vigilance states, the delta activity and EEG energy over the light- and dark-period.