While the brain sleeps, it clears out harmful toxins, a process that may reduce the risk of Alzheimer's, researchers say.

During sleep, the flow of cerebrospinal fluid in the brain increases dramatically, washing away harmful waste proteins that build up between brain cells during waking hours, a of mice found.

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The results appear to offer the best explanation yet of why animals and people need sleep. If this proves to be true in humans as well, it could help explain a mysterious association between sleep disorders and brain diseases, including Alzheimer's

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The brain-cleaning process has been observed in rats and baboons, but not yet in humans, Nedergaard says. Even so, it could offer a new way of understanding human brain diseases including Alzheimer's. That's because one of the waste products removed from the brain during sleep is beta amyloid, the substance that forms sticky plaques associated with the disease.