Bilateral flow patterns of the middle cerebral artery (MCA) were recorded continuously throughout the night in 18 healthy male subjects (mean age 27.4 years) by a computer-assisted pulsed Doppler (2 MHz) system together with simultaneous polysomnography. After inception of sleep, mean flow velocity (MFV) decreased steadily during deepening sleep stages reaching -15.0 +/- 3.6% (p < 0.001) in the right MCA and -16.2 +/- 3.4% in the left MCA (p < 0.001) in stage 4 of the first sleep cycle compared to the waking state. Lowest MFV values were found in stage 2 preceding the last REM period (right MCA: -19.2 +/- 4.1%: left MCA: -19.7 +/- 5.1%). Changing from non-REM into REM sleep, a sudden rise of MFV, which varied from 8.9% (first sleep cycle, left MCA) to 18% (last sleep cycle, right MCA), could be consistently detected indicating a coupling of cerebral electrical activity and cerebral perfusion in REM sleep. During non-REM sleep this concomitant change of MFV and EEG activity was only found in the first sleep cycle, whereas no parallel changes could be observed in later sleep cycles. These results indicate a decoupling of EEG measured cerebral electrical activity and perfusion and suggest that factors other than metabolic mechanisms contribute to the regulation of cerebral perfusion during human non-REM sleep.