Sleep keeps the brain healthy. That much has been known for centuries. But how? And why are people who sleep poorly more susceptible to Alzheimer's disease and other brain disorders?
The answer appears to involve a recently discovered sewage system that flushes old proteins and other debris from the brain. During sleep and anesthesia the rate of clearance doubles, which means poor sleep allows debris to accumulate, including the toxic proteins believed to trigger Alzheimer's disease.
"We need sleep; it cleans up the brain," said Dr. Maiken Nedergaard, the lead author of a paper in the Oct. 18 issue of Science that explains the process.
The process helps solve one of the greatest mysteries about the brain: How does it get rid of the waste without help from the lymphatic system, which clears debris from the rest of the body?
Lymph fluid carries away waste very efficiently, but it cannot enter the brain, even though the brain consumes about 20 percent of the oxygen carried in the blood, and produces enormous amounts of metabolic waste. So how does the brain dispose of all that waste?
Dr. Nedergaard and colleagues at the University of Rochester Medical Center in Rochester, N.Y., drilled a hole in the skull of mice and inserted a tiny lens attached to a microscope. Then they injected dye into the cerebral spinal fluid of the mice and watched it flow through the brain, carrying waste away. They found that when the mice were asleep or under anesthesia, the rate of flow doubled, presumably removing waste twice as quickly.
They also found that the "glymphatic system," as Nedergaard has named it, removes the toxic protein fragments known as beta-amyloid, believed to trigger Alzheimer's.
The relationship between brain flushing and Alzheimer's reported in the Science paper received support from another study published at almost exactly the same time.
That research, published in JAMA Neurology, found a link between poor sleep and signs of approaching Alzheimer's disease.
But does poor sleep promote Alzheimer's, or does early Alzheimer's cause poor sleep?
"Probably both," said lead author Adam Spira, a professor at the Johns Hopkins Bloomberg School of Public Health. "My best guess is that sleep disturbances begin a cascade that may very well contribute to Alzheimer's disease and other neurological diseases, but there's also evidence that the brain pathology causes a decline in sleep quality."
So what to do? If you snore or have other symptoms of sleep apnea, go to the doctor and get tested in a sleep lab. If the results show you stop breathing many times during the night, wear a C-PAP mask to help you breathe normally as you sleep.
If you have difficulty falling asleep, or if you wake too soon after falling asleep, see a sleep specialist who can help you improve your "sleep hygiene," which involves lifestyle changes, such as consuming less caffeine, not taking naps during the day, and avoiding TV and computer screens before going to bed.
Tom Valeo writes on health matters. He can be reached at firstname.lastname@example.org.