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 study of mice found.

“It’s like a dishwasher,” says Dr. Maiken Nedergaard, a professor of neurosurgery at the University of Rochester and an author of the study in Science.

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.

Nedergaard and a team of scientists discovered the cleaning process while studying the brains of sleeping mice.

The scientists noticed that during sleep, the system that circulates cerebrospinal fluid through the brain and nervous system was “pumping fluid into the brain and removing fluid from the brain in a very rapid pace,” Nedergaard says.

The team discovered that this increased flow was possible in part because when mice went to sleep, their brain cells actually shrank, making it easier for fluid to circulate. When an animal woke up, the brain cells enlarged again and the flow between cells slowed to a trickle. “It’s almost like opening and closing a faucet,” Nedergaard says. “It’s that dramatic.” To read more from JON HAMILTON, click here.