One More Reason to Sleep: Brain is Flushed of Toxins While at Rest

Everybody knows the importance of a good night's sleep, but new research suggests that sleeping literally clears the mind by allowing the space between brain cells to expand, allowing toxins that build up over the waking hours to flush out more easily.

Writing in the journal Science, lead study author Maiken Nedergaard said that sleep changes the cellular structure of the brain.

"It appears to be a completely different state," said Nedergaard who is the co-director of the Center for Translational Neuromedicine at the University of Rochester Medical Center in New York.

Recent research has suggested that sleep is important for storing memories. Building on that research, Nedergaard and her colleagues continued to analyze the brain during states of sleep, unexpectedly finding that the brain clears itself of toxic molecules while the body is asleep.

During sleep, the researchers suggest, the brain's glymphatic system -- a sort of plumbing system -- may open up, allowing fluid to flow rapidly through the brain. Nedergaard's work in the lab suggests that the glymphatic system also helps control the flow of cerebrospinal fluid, a clear liquid surrounding the brain and spinal cord.

The results may indicate a previously unknown need for sleep in the treatment and study of health and disease.

"It's as if Dr. Nedergaard and her colleagues have uncovered a network of hidden caves and these exciting results highlight the potential importance of the network in normal brain function," said Roderick Corriveau, a program director at National Institute of Neurological Disorders and Stroke (NINDS), part of the National Institutes of Health. The NIH funded Nedergaard's research.

For the study, Nedergaard and her team injected dye in between the cells of the brains of mice and watched it flow as they monitored the mice's brain electrical activity.

When the mice were asleep or unconscious, the dye flowed rapidly. But when the mice were awake, the dye barely flowed at all.

"We were surprised by how little flow there was into the brain when the mice were awake," Nedergaard said. "It suggested that the space between brain cells changed greatly between conscious and unconscious states."

When the mice were asleep or anesthetized, the space between their brain cells increased by 60 percent, which Charles Nicholson, an expert in measuring the dynamics of brain fluid flow, said was a "dramatic" change in extracellular space.

"These results may have broad implications for multiple neurological disorders," said Jim Koenig, a program director at NINDS. "This means the cells regulating the glymphatic system may be new targets for treating a range of disorders."

The study also highlights the simple importance of sleep.

"We need sleep. It cleans up the brain," Nedergaard said.