The study was aimed at the examination of the acute and cumulative impact of partial sleep deprivation (PSD) on architecture and quality of sleep, on circadian rhythm and on daytime fatigue.Time in bed for 16 healthy male volunteers was reduced from 8 to 5h during four consecutive nights, followed by two recovery nights. This scheme matches the terms of service and sleep rhythms of rescue helicopter pilots in Germany. Polysomnography was recorded during each night, and motor activity and body core temperature were recorded continuously. Subjective sleep quality and alertness were acquired by questionnaires.In the course of PSD, electroencephalography (EEG) showed a reduction of S1 and S2 (p=0.039), whereas S3, S4 (0.024) and rapid eye movement (REM; p=0.030) sleep increased. Subjective sleep quality improved, while sleep need (p<0.001) and fatigue (p<0.001) deepened. These effects declined rapidly after one recovery night.PSD alters sleep and daytime alertness. The results indicate that not only slow wave sleep (SWS) but also REM is important for sleep. Sleep became more effective when wake periods and sleep latencies decreased, thereby improving subjective sleep quality. Sleepiness, sleep need and fatigue increased immediately and accumulated throughout the restricting procedure.