A new study offers an explanation for why a lack of sleep can make us feel as though we’re walking around in a fog. While we’re asleep, our bodies may be resting, but our brains are busy cleaning house.
During sleep, the flow of cerebrospinal fluid in the brain increases dramatically, washing away harmful waste proteins that build up between brain cells during waking hours. “It’s like a dishwasher,” says Dr. Maiken Nedergaard, a professor of neurosurgery at the University of Rochester. Dr. Nedergaard and her team published their results last Thursday in the journal, Science.
It’s a revelation that could not only transform scientists’ fundamental understanding of sleep, but also point to new ways to treat disorders such as Alzheimer’s disease, which are linked to the accumulation of toxins in the brain. “We have a cleaning system that almost stops when we are awake and starts when we sleep. It’s almost like opening and closing a faucet — it’s that dramatic,” says Dr. Nedergaard.
According to study authors, sleep puts the brain in another state where we clean out all the by-products of activity during the daytime. Those by-products include beta-amyloid protein, clumps of which form plaques found in the brains of Alzheimer’s patients. Staying up all night could prevent the brain from getting rid of these toxins as efficiently, and explain why sleep deprivation has such strong and immediate consequences. Too little sleep causes mental fog, crankiness, and increased risks of migraine and seizure. Previous studies have shown that rats deprived of all sleep die within weeks.
Nedergaard and her colleagues first reported their discovery of the brain’s unique waste removal system, dubbed the “glymphatic system” last year. It works like a neural trash truck, clearing away toxic by-products that build up when you’re awake. The scientists used two-photon microscopy — a new imaging technology that allows scientists to see deep inside living tissue — to peer into the brains of mice, which are remarkably similar to human brains.
They found that the glymphatic system pumps cerebral spinal fluid, CSF, through the spaces around the brain cells, flushing waste into the circulatory system, where it eventually makes its way to the liver.
Their latest research, also in mice, used the same technology to focus on the timing of the glymphatic system. The researchers discovered that during sleep brain cells contract, increasing the space between the cells by as much as 60 percent and allowing the spinal fluid to wash more freely through the brain tissue. When the mouse woke up, the brain cells enlarged again and the flow between cells slowed to a trickle.
People with diseases that cause progressive brain decline such as Alzheimer’s and Parkinson’s often sleep poorly. The diseases are also associated with the abnormal buildup of protein in the brain. While researchers don’t yet know if these plaques are a cause or a result of neurodegenerative disease, the new insights about the way sleep clears waste from the brain could lead to new treatment approaches.
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