Sleep homeostasis and slow oscillations in the human sleep EEG

Sleep homeostasis is a basic principle of sleep regulation. Slow-wave activity (spectral power in the 0.7-4.5 Hz range of the EEG) is a well-established corresponding indicator. Prominent features of the non-rapid-eye-movement sleep EEG are slow oscillations resulting from slow membrane potential fluctuations of cortical neurons. We investigated the behaviour of slow oscillations in humans under increased sleep pressure to assess their contribution to sleep homeostasis. We analyzed baseline and recovery sleep after 40 h of prolonged wakefulness. Negative half-waves, mainly reflecting down states, were defined as negative deflections between consecutive zero crossings in the 0.5-2 Hz range of the band-pass filtered EEG. Wave parameters (amplitude, number of half-waves per min and slope) were derived for two thresholds: a low threshold allowing the detection of approximately all negative half-waves and a higher one corresponding to the scoring rules of slow wave sleep. The number of detected high amplitude waves and amplitude and slope of all waves closely resembled slow-wave activity, indicating that slow oscillations contribute to sleep homeostasis.