Among the many vital roles that sleep plays in our lives, our nightly rest may give us the chance to take out the cerebral trash, says a new study.
No, we're not talking about some kind of Ambien-induced sleep-housework. We're talking about the process by which the brain refreshes itself by removing the buildup of mental metabolites such as beta-amyloid and tau -- the byproducts, if you will, of a day's cogitation.
Left to fester on the sidewalks of our brains, these byproducts of everyday mental activity can gum up the works in a hurry. The hum of electrical signals across synapses slows, and neurons can give up and die in the foul environment of unmanaged neural trash.
Accumulations in the brain of tau and beta-amyloid are hallmarks of certain dementias, including Alzheimer's Disease. And perhaps not so coincidentally, sleep problems often precede the onset of obvious dementia symptoms, and are a continuing problem for those afflicted with such diseases as Alzheimer's.
But although brain cells burn up a vast amount of fuel and are highly sensitive to a buildup in their own metabolites, the brain has a trash-removal process that is far less straightforward than that by which wastes are removed from the rest of the body.
The lymph system collects metabolites from tissues throughout the body and dumps them into the bloodstream, where they're carried to the liver for breakdown and removal. The brain's metabolic waste concentrates in interstitial fluid present in all corners of the brain. A second slurry -- cerebrospinal fluid -- circulates throughout the brain, and where the two fluids flow together, the metabolic byproducts are carried away by the cerebrospinal fluid.
In a new study, scientists from University of Rochester Medical Center and New York University found that the brains of mice -- whether they are sleeping or anesthetized -- showed more activity and volume at the "transfer stations," where interstitial and cerebrospinal fluid meet, than did mice who were awake and active. The result was that by the end of a sleep period -- around early evening -- mouse brains had their lowest concentration of neural refuse of the day. By the time they were ready to sleep again, those concentrations had reached their peak.
It wasn't just the mouse circadian schedule that initiated the trash removal: Even when researchers used the powerful sedative ketamine to put the mice to sleep, they saw evidence of a sudden increase in traffic at the brain's transfer stations.
Noting the link between sleep deprivation or disruption and neurodegenerative disease, the authors suggest that neural trash removal must be one of sleep's major benefits. Indeed, they surmised, it could even be that the buildup of brain refuse may be one of the cues that drives us to bed, and that an empty trash bin may help signal us to wake and initiate another day of mental activity and its inevitable byproduct, brain trash.