EEG power density during recovery sleep in the morning.

Sleep was recorded under baseline conditions (waking prior to sleep 16 h; lights off 23.00 h) and during recovery sleep in the morning (waking prior to sleep 24 h; lights off 07.00 h). Slow-wave activity (SWA; EEG power density in the range of 0.75-4.5 Hz) declined progressively over consecutive nonREM-REM cycles in both conditions despite the different circadian phase at which sleep occurred. SWA in nonREM sleep in the first 5 h of sleep was significantly higher in recovery than in baseline. Also SWA within the first 20 min of nonREM-episodes 2 and 3 was significantly higher in recovery sleep, and a tendency in the same direction was seen for nonREM-episode 1. These data show that homeostatic processes are expressed in the EEG also when sleep is initiated at a circadian phase where REM sleep propensity is high. However, comparison of the power spectrum in the first cycle of day-time recovery sleep with published data on recovery sleep at various circadian phases suggests that circadian factors influence the EEG spectra.