The purpose of sleep has long puzzled scientists, but a new study may have identified its purpose: Sleep cleanses the brain of damaging molecules associated with the loss of brain function, the same toxins that contribute to Alzheimer's disease.
For the study—published Friday in Science Translational Medicine—University of Rochester Medical Center researchers injected dyed beta-amyloid (a protein associated with Alzheimer's) into the cerebrospinal fluid of mice. They found that the brain's glymphatic system—its "plumbing system"—opens during sleep and allows fluid to flow rapidly through the brain. But when awake, the fluid barely flows.
"We were surprised by how little flow there was into the brain when the mice were awake," says Maiken Nedergaard, co-director of Rochester's Center for Translational Neuromedicine, adding that the beta-amyloid disappeared quickly when the mice were asleep.
Researchers then inserted electrodes into the mice brains to directly measure the space between brain cells. They found that the space inside the brains increased by 60% when the mice were asleep—the space allowed toxins to be flushed away more effectively, researchers say.
"This study shows that the brain has different functional states when asleep and when awake. 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," Nedergaard said.