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Summary
Getting enough quality sleep at the right times helps protect physical and mental health. Slow-wave sleep (SWS) represents high-intensity sleep and predominates in the first part of the night. Rapid eye movement (REM) sleep accompanies dreaming and is not influenced by prior sleep or waking and shows a rhythmicity with the body clock (circadian rhythm) (1): Sleep starts with a light form of SWS, progresses to deeper SWS and then shallow SWS, and concludes with REM sleep before beginning a new cycle (2). During SWS, the cerebral blood flow is reduced by 25%, which lowers cerebral blood volume (CBV) by ∼10% (3). On page 628 of this issue, Fultz et al. (4) show that this reduction in CBV in the human brain allows inflow of cerebrospinal fluid (CSF) to the third and fourth ventricles—fluid-filled cavities in the central brain—which may facilitate communication between fluid compartments and clearance of waste products.
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