For several years, researchers have known non-REM sleep plays a key role in clearing the brain of toxins that buildup during the day. But a new study from Boston University last year helped us better understand how this process occurs, Psychology Today reports.

What the researchers found was that during non-REM sleep, slow waves of cerebrospinal fluid, a clear, watery liquid, would essentially wash over the brain. This process occurred when neurons started to sync up during non-REM sleep, turning off and on in unison; when the neurons weren’t firing, they didn’t need as much oxygen as usual, meaning less blood would enter into the brain. 

That’s when cerebrospinal fluid would come rushing in and clear out metabolic buildup, like beta-amyloid. Other proteins and molecules that accumulate during the day would also be cleared out; this includes tau, a protein associated with Alzheimer’s disease because it stands in the way of neurons connecting properly.

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