Though the cleaning was observed in mice, scientists think it happens in people too.
The findings were reported in Friday's issue of the journal Science and may provide new clues to treat Alzheimer's and other brain diseases.
People who are sleep-deprived have trouble learning and making decisions. But despite decades of research, scientists can't agree on the basic purpose of sleep.
For the study, researchers led by the University of Rochester Medical Center observed the brains of mice while awake and asleep. They found cellular waste flowed faster out of the brains of sleeping mice than awake mice.
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