Scientists filmed the brain washing itself during sleep

Researchers at Boston University and Massachusetts General Hospital used a fast form of functional MRI, sampling the brain many times each second, to track neural activity, blood flow and fluid movement together in people during non-REM sleep.

Large, slow brain waves appeared first, then a pulse of blood flow a moment later, then a wave of cerebrospinal fluid rushing into the brain's fourth ventricle, all locked to the same rhythm, once every twenty seconds or so, during non-REM sleep.

The pattern looked like a mechanical pump, powered by the brain's own electrical rhythm, pushing fluid in a way that could help carry waste, including the proteins that build up in Alzheimer's disease, out of the brain. The study involved healthy young sleepers, so whether the same pump weakens with age or disease is still being tested.

Sleep runs its own cleaning cycle.