The US team believes the “waste removal system” is one of the fundamental, physical reasons for sleep.
Their study, in the journal Science, showed brain cells shrink during sleep to open up the gaps between neurons and allow fluid to wash the brain clean.
They also suggest that failing to clear away some toxic proteins may play a role in brain disorders.
Their findings build on last year's discovery of the brain's own network of plumbing pipes - known as the glymphatic system - which carry waste material out of the brain.
Dr Nedergaard said this was a "vital" function for staying alive, but did not appear to be possible while the mind was awake.
Dr Neil Stanley, an independent sleep expert, said: "There is good data on memory and learning, the psychological reason for sleep. But this is the actual physical and chemical reason for sleep, something is happening which is important."
The researchers suggest that problems with the brain's cleaning mechanism may contribute to such diseases, but caution more research is needed.
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http://www.bbc.co.uk/news/health-24567412
