Dr. Maiken Nedergaard, co-director of the Center for Translational Neuromedicine at the University of Rochester Medical Center in New York, and colleagues said they unexpectedly found sleep might be the period when the brain cleanses itself of toxic molecules.
The study, published in Science, found a plumbing system called the glymphatic system might open and let fluid flow rapidly through the brain during sleep.
Nedergaard's lab recently discovered the glymphatic system helps control the flow of cerebrospinal fluid, a clear liquid surrounding the brain and spinal cord.
Initially dye was injected into the cerebrospinal fluid of mice and he researchers watched it flow through their brains while simultaneously monitoring electrical brain activity.
The dye flowed rapidly when the mice were unconscious, either asleep or anesthetized, but barely flowed when the same mice were awake, Nedergaard said.
"We were surprised by how little flow there was into the brain when the mice were awake," Nedergaard said in a statement. "It suggested that the space between brain cells changed greatly between conscious and unconscious states."
To test this, the researchers inserted electrodes into the brain to directly measure the space between brain cells and found the space inside the brains increased by 60 percent when the mice were asleep or anesthetized.
Previous research suggested toxic molecules involved in neurodegenerative disorders accumulate in the space between brain cells. In this study, the researchers tested whether the glymphatic system controlled this by injecting mice with beta-amyloid, a protein associated with Alzheimer's disease. They measured how long it lasted in their brains when they were asleep or awake.
Beta-amyloid disappeared faster in mice brains when the mice were asleep, suggesting sleep normally clears toxic molecules from the brain, Nedergaard said.
The results may also highlight the importance of sleep.
"We need sleep," Nedergaard said. "It cleans up the brain."