Brain's Waste Removal System More Active During Sleep

Even our brains need to take out the trash.

Researchers from the University of Rochester Medical Center found that a waste-flushing system in the brain, called the glymphatic system, is most active when we sleep -- nearly 10 times more so than during periods of wakefulness, in fact.

Plus, during sleep, brain cells shrink in size by 60 percent to better allow for the removal of waste from the brain.

"This study shows that the brain has different functional states when asleep and when awake," study researcher Maiken Nedergaard, M.D., D.M.Sc., the co-director of the university's Center for Translational Neuromedicine, said in a statement. "In fact, the restorative nature of sleep appears to be the result of the active clearance of the by-products of neural activity that accumulate during wakefulness."

The findings, which are published in the journal Science, are based on brain imaging experiments in mice, using technologies such as two-photo microscopy. Researchers found that when the mice were put to sleep, cerebral spinal fluid is ushered through their brains through this waste-removal system, which then moves toxins to the circulatory system and the liver.

"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," Nedergaard added in the study. "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."

Other things that can occur during sleep? Reduction of fears, consolidation of memories, the learning of new motor tasks, and repair and growth of brain cells through the formation of myelin.