According to a press release from the National Institute of Neurological Disorders and Stroke, utilizing mice, researchers showed for the very first time that the space between brain cells may actually increase while we sleep, allowing the brain to flush away toxins that build up during our waking hours. These results suggest an entirely new role for sleep in health and disease.

“Sleep changes the cellular structure of the brain. It appears to be a completely different state,” said Maiken Nedergaard, M.D., D.M.Sc., co-director of the Center for Translational Neuromedicine at the University of Rochester Medical Center in New York.

Scientists and philosophers both have wondered for centuries why we need to sleep and how it affects our brain. Only recently have scientists been able to show that sleep is important for storing memories. In this most recent study, Dr. Nedergaard and her colleagues unexpectedly discovered that sleep might also be when the brain rids itself of toxic molecules accumulated during waking hours.

The study results show that while we sleep a system called the glymphatic system may open, letting fluid flow rapidly through the brain. Dr. Nedergaard’s lab recently discovered the glymphatic system helps control the flow of cerebrospinal fluid (CSF), a clear liquid that surrounds the brain and spinal cord.

“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, Ph.D., a program director at NINDS.

Initially researchers were studying the system by injecting dye into the CSF of mice and watching it flow through their brains while monitoring their electrical brain activity. The dye flowed rapidly when the mice were unconscious, either asleep or anesthetized. However, the dye barely flowed when the very same mice were awake.

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

To test their idea, researchers used electrodes inserted into the brain to directly measure the space between brain cells. It was then that they discovered the space between the brain cells increased by 60 percent when the mice were asleep or anesthetized.

This study was funded by the National Institute of Neurological Disorders and Stroke (NINDS) and published in the journal Science.