Next time someone advises you to “sleep on it,” consider doing just that. Research published Thursday suggests that sleep may literally clean up the brain.

“The brain only has limited energy at its disposal and it appears that it must choose between two different functional states — awake and aware or asleep and cleaning up,” said study author Maiken Nedergaard, co-director of the University of Rochester Medical Center for Translational Neuromedicine. “You can think of it like having a house party. You can either entertain the guests or clean up the house, but you can’t really do both at the same time.”

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The study, published in the journal Science, also shows that cells seem to shrink during sleep — by as much as 60 percent. That makes more room for cerebral spinal fluid to flow through brain tissue, which flushes waste into the circulatory system. That waste removal system, called the glymphatic system, is important in understanding neurological disorders such as Alzheimer’s disease because it clears away toxins associated with those conditions.

In their experiments in mice, researchers found that the glymphatic system was almost 10 times more active during sleep. That could account for why the brain uses almost as much energy during sleeping hours as waking hours, the researchers said.

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The researchers also found that one hormone, noradrenaline, is less active in sleep. They suspect that the neurotransmitter, which causes the brain to become alert in response to stimuli such as fear, may control the contraction and expansion of the brain’s cells during sleep-wake cycles.

“These findings have significant implications for treating ‘dirty brain’ disease like Alzheimer’s,” Nedergaard said. “Understanding precisely how and when the brain activates the glymphatic system and clears waste is a critical first step in efforts to potentially modulate this system and make it work more efficiently.”


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