As we learn more about potential ways to ward off dementia and Alzheimer's disease as we age, from exercise to diet to web surfing to marijuana use, a new study makes the case that getting a good night's sleep just might be the most important thing we can do.
Our brain cells produce toxic waste products each day as they work. The new study, published this week in the journal Science, shows that while we sleep, the brain literally flushes out this gunk. The self-cleaning process, which scientists observed in resting mice, is a powerful illustration of the medical importance of sleep. Researchers had suspected that this self-cleaning went on in our heads each night, but the new study put the process, and its intensity, in far clearer focus. For example, the team witnessed that when the mice slept, brain cells actually shrunk in size, expanding the spaces in between them by as much as 60 percent and facilitating the flushing of waste.
"It's like opening and closing a faucet," said University of Rochester neurosurgeon Maiken Nedergaard, who directed the study.
At minimum, the research highlights the potential importance of regular sleep in slowing dementia, as well as the possible neurological risks of consistently getting too little sleep. When we stay up until late into the night, we may be preventing our brains from flushing toxins effectively. This may also explain why we can feel uncertain or cranky when we are sleep-deprived and perhaps why migraines and seizures appear to be exacerbated by poor rest.
A year ago, Nedergaard's team identified the network for flushing waste from the brain and named it the glymphatic system. During this cleansing, cerebrospinal fluid circulates through brain tissue, carrying waste matter into the bloodstream toward the liver, where it is detoxified. Similar systems, she noted, have been detected in the brains of dogs and baboons. Neuroscientists now widely assume that this self-cleaning takes place in humans as well, but the next step will be to directly observe the process.
A New Window on Sleep
Scholars have long wondered about the biological purpose of sleep. The idea that we sleep to conserve energy has been somewhat debunked; studies have found that the brain uses almost as much energy at rest as it does when we're awake. Another theory held that a full night's sleep was necessary to lock in memories, but as Nedergaard and others have pointed out, seven or eight hours appears to be excessive for this purpose, given what we now know about the speed of human memory processing.
A body of research does connect consistent sleep to the maintenance of human metabolism, which is why experts typically recommend that people trying to lose weight always get a full night's sleep. But the new study indicates that a primary reason for sleep, and the reason it feels so restorative, is that we awake with the remains of the previous day's activity cleared from our heads.
A Step Toward an Alzheimer's Treatment?
To observe the glymphatic system in mice, the research team injected rodents with beta-amyloid, a protein that builds up in clumps in the brains of Alzheimer's patients, forming plaques. By tracking the animals' brains in real time using an imaging process known as two-photon microscopy, they were able to watch fluid move between cells. Researchers found that waste was flushed out of the brain cells of sleeping mice twice as fast as in those of conscious mice. "It was almost like you opened a faucet," Nedergaard said.
Experts expressed hope that the new findings could lead to treatments for neurological ailments associated with cell waste in the brain, including Parkinson's disease as well as Alzheimer's and dementia. Scientists will be following up on the tantalizing possibility that Alzheimer's is exacerbated not as much by the buildup of beta-amyloid plaque in the brain as by an impaired ability to flush it out. If that turns out to be true, then the development of a drug to facilitate or force the self-cleaning process could be a major breakthrough. Doctors may also achieve better results by coordinating dementia patients' treatments with their sleep schedules.
"I'd be a fool not to pay attention to this," Washington University neuroscientist Randall Bateman, an expert in amyloid-beta research, told the Science News blog.