Slow oscillating neural activity during non-REM sleep triggers waves of cerebrospinal fluid that flow in and out of the sleeping brain, washing it of harmful metabolic waste products, researchers find. Their new study reveals a key component in the neurophysiology of sleep. Since slow-wave sleep disruption and the buildup of toxic metabolic proteins in the brain have been linked to several diseases, including Alzheimer's, the results could help to improve disease outcomes. Sleep is crucial for a healthy brain. The slow-wave neuronal activity of Non-REM (NREM) sleep is known to support both memory processing and consolidation, as well as the physiological maintenance of the brain. During NREM sleep, increased volume and flow of interstitial fluid wash over the brain and flush out accumulating toxic waste proteins that can lead to neurodegeneration. However, why these two seemingly unrelated processes occur simultaneously, or if they are even linked, remains unexplained. Nina Fultz and colleagues used accelerated neuroimaging techniques to measure the physiological and neural activity in 11 sleeping human brains and revealed a pattern between electrophysiological, blood and cerebral spinal fluid dynamics during NREM sleep. According to the results, neuronal slow waves are followed by oscillations in blood volume and flow in the brain, which creates space for coupled waves of cerebral spinal fluid to flow in and out of the brain cavity. Fultz et al. show that neuronal slow waves are not just important for memory - they also alter the physiological and fluid dynamics of the brain. In a related Perspective, Søren Grubb and Martin Lauritzen discuss the findings and their implications in more detail.