Researchers found that during deep sleep, the "slow-wave" activity of nerve cells appears to make room for cerebral spinal fluid to rhythmically move in and out of the brain - a process believed to rinse out metabolic waste products.
Those waste products include beta-amyloid - a protein that clumps abnormally in the brains of people with dementia, said researcher Laura Lewis, an assistant professor of biomedical engineering at Boston University.
Lewis stressed that the findings, reported in the Nov. 1 issue of Science, do not prove that deep sleep helps ward off dementia or other diseases.
But the ultimate goal of research like this is to understand why poor sleep quality is linked to higher risks of various chronic conditions, from dementia to heart disease to depression, she said.
Researchers have known that cerebral spinal fluid, or CSF, helps clear metabolic byproducts from the brain, so that they do not build up there. They've also known that the process appears to amp up during sleep. But various "hows" and "whys" remained.
So the investigators recruited 11 healthy adults for a sleep study using noninvasive techniques: advanced MRI to monitor fluid flow in the brain, and electroencephalograms to gauge electrical activity in brain cells.
Sleep is marked by REM and non-REM cycles. During REM sleep, breathing and heart rates are relatively higher, and people often have vivid dreams. Non-REM sleep includes stages of deep - or slow-wave - sleep. During those stages, there's a slow-down in brain cell activity, heart rate and blood flow, and research has found that deep sleep may aid memory consolidation and allow the brain to recover from the daily grind.
"There are all these fundamental things your brain is taking care of during deep sleep," Lewis said.
Her team found that housecleaning may be one. When study participants were in deep sleep, each pulse in slow-wave brain activity was followed by oscillations in blood flow and volume, which allowed CSF to flow into fluid-filled cavities in the central brain.
CSF moved in "large, pulsing waves" that were seen only during deep sleep, Lewis explained.
Based on what's known about the work of CSF, experts said it's reasonable to conclude that slow-wave sleep promotes the flushing of waste from the brain.
The study "elegantly" illustrates the importance of deep sleep, according to Dr. Phyllis Zee, a sleep medicine specialist not involved in the work.
It "helps to explain how and why sleep is important for keeping neurons healthy - facilitating the removal of toxic molecules," said Zee, a professor of neurology at Northwestern University Feinberg School of Medicine, in Chicago.
"One can think of sleep as a top way to take care of your brain," she said.
Another sleep medicine specialist agreed. "There is growing evidence, with this study and others, that sleep plays a role in clearing toxins from the brain," said Dr. Raman Malhotra, an associate professor of neurology at Washington University in St. Louis.
Other research has suggested that sleep loss can promote the buildup of "unwanted proteins" in the brain, said Malhotra, who also serves on the board of directors of the American Academy of Sleep Medicine.
A recent government study, for instance, found that one night of sleep deprivation triggered an increase in beta-amyloid in the brains of healthy adults.
"As we learn more about this role of sleep," Malhotra said, "it may help explain why individuals who don't get enough sleep, or suffer from sleep disorders, are at higher risk of certain chronic health conditions."
The latest study involved younger adults with no health problems. Lewis said that it will be important to find out whether healthy older adults, or people with certain health conditions, show any differences in CSF dynamics during deep sleep.
A big question for future research, she said, will be whether alterations in those dynamics precede the development of disease.
Sources: Laura Lewis, Ph.D., assistant professor, biomedical engineering, Boston University; Phyllis Zee, M.D., Ph.D., professor, neurology, and chief, sleep medicine, Northwestern University Feinberg School of Medicine, Chicago; Raman Malhotra, M.D., associate professor, neurology, Washington University School of Medicine, St. Louis, and secretary/treasurer, American Academy of Sleep Medicine board of directors; Nov. 1, 2019, Science.
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