Progress in brain research
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Sleep is regulated by circadian and homeostatic processes. The sleep homeostat keeps track of the duration of prior sleep and waking and determines the intensity of sleep. In mammals, the homeostatic process is reflected by the slow waves in the non-rapid eye movement (NREM) sleep electroencephalogram (EEG). ⋯ Under similar conditions, humans show a clear circadian modulation in REM sleep, whereas in the rat, a circadian modulation in REM sleep was not present. Therefore, in the rat, the sleep homeostatic modulation in phase with the circadian clock seems to amplify the relatively weak circadian changes in sleep induced by the circadian clock. Knowledge about the interaction between sleep and the circadian clock and the circadian modulation of sleep in other species than humans is important to better understand the underlying regulatory mechanisms.