In a sleep loss experiment with 8 subjects night sleep was restricted to either 8, 4, 2 or 0 h. The restrictions caused a strong dose-dependent increase in the duration of subsequent day sleep. Still, only a fraction of the sleep loss was recovered. Spectral analysis of the EEG demonstrated a strong dose-dependent increase also of power density. This parameter showed an almost perfect match between loss and recovery. Furthermore, daytime sleep duration was closely determined by the prior loss of EEG power density. Day sleep was maintained until the prior loss of power density had been recovered, at which point the sleep episode was spontaneously terminated. It was suggested that EEG power density reflects the homeostatic process in sleep regulation and that this process is a major determinant of sleep duration and content.