We used the waking electroencephalogram to study the homeostatic sleep regulatory process in human short sleepers and long sleepers. After sleeping according to their habitual schedule, nine short sleepers (sleep duration < 6 h) and eight long sleepers (> 9 h) were recorded half-hourly during approximately 40 h of wakefulness in a constant routine protocol. Within the frequency range of 0.25-20.0 Hz, spectral power density in the 5.25-9.0 and 17.25-18.0 Hz ranges was higher in short sleepers than in long sleepers. In both groups, increasing time awake was associated with an increase of theta/low-frequency alpha activity (5.25-9.0 Hz), whose kinetics followed a saturating exponential function. The time constant did not differ between groups and was similar to the previously obtained time constant of the wake-dependent increase of slow-wave activity (0.75-4.5 Hz) in the sleep electroencephalogram. In addition, the time constant of the decrease of slow-wave activity during extended recovery sleep following the constant routine did not differ between groups. However, short sleepers showed an abiding enhancement of theta/low-frequency alpha activity during wakefulness after recovery sleep that was independent of the homeostatic process. It is concluded that, while the kinetics of the homeostatic process do not differ between the two groups, short sleepers live under and tolerate higher homeostatic sleep pressure than long sleepers. The homeostat-independent enhancement of theta/low-frequency alpha activity in the waking electroencephalogram in the short sleepers may be genetically determined or be the result of long-term adaptation to chronically short sleep.