Slow oscillations (< 1 Hz) in the non-rapid eye movement (NREM) sleep electroencephalogram (EEG) result from slow membrane potential fluctuations of cortical neurones, alternating between a depolarized up-state and a hyperpolarized down-state. They are thought to underlie the restorative function of sleep. We investigated the behaviour of slow oscillations in humans under increased sleep pressure to assess their contribution to sleep homeostasis. EEG recordings (C3A2) of baseline and recovery sleep after sleep deprivation (eight healthy males, mean age 23 years; 40 h of prolonged wakefulness) were analysed. Half-waves were defined as positive or negative deflections between consecutive zero crossings in the 0.5-2 Hz range of the band-pass filtered EEG. Increased sleep pressure resulted in a redistribution of half-waves between 0.5 and 2 Hz: the number of half-waves per minute was reduced below 0.9 Hz while it was increased above 1.2 Hz. EEG power was increased above 1 Hz. The increase in frequency was accompanied by increased slope of the half-waves and decreased number of multi-peak waves. In both baseline and recovery sleep, amplitude and slope were correlated highly over a broad frequency range and positive half-waves were characterized by a lower frequency than the negative ones, pointing to a longer duration of up- than down-states. We hypothesize that the higher frequency of slow oscillatory activity after prolonged wakefulness may relate to faster alternations between up- and down-states at the cellular level under increased sleep pressure. This study does not question slow-wave activity as a marker of sleep homeostasis, as the observed changes occurred within the same frequency range.