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For a long time, researchers weren’t quite sure why we sleep. There are a number of different theories. Some are evolutionary: For instance, sleep keeps us out of trouble at night and away from animals who wake up and hunt after the sun goes down. Some are physiological: Sleep lets us conserve precious energy. And some are anecdotal: Parents joke that they’d go nuts if sleep didn’t exist, since it affords them a much-needed break from their kids. But the reality is that the brain does a lot of work while we’re sleeping – even though we’re unconscious, this doesn’t at all mean the brain is “off.” In fact, in many ways, it's incredibly "on."
The RAND research group just came out with a 100-page analysis of how sleep affects us and what sleep deprivation can do to us – and to the economy. They estimate that between lost work and poor performance at work from lack of sleep, the U.S. alone loses $411 billion each year. Though businesses and policy makers may be interested in the financial repercussions of sleep deprivation, these repercussions stem from people being unwell because of it, which underlines the very real consequences of sleep deprivation.
Recent research has laid out some of the reasons why we need sleep, and all the functions the brain seems to perform while we’re sleeping. There's more to figure out, but here are a few reasons why the brain needs sleep, and why things tend to go downhill without it.
Sleep helps solidify memory
One of the central functions of sleep is that it helps consolidate long-term memory – it seems to do this, not only through strengthening certain neural connections, but also through pruning back unwanted ones. The brain makes a lot of connections during the day, but not all of them are worth saving; so sleep is a time in which the brain streamlines the connections it “needs.”
Most people have probably observed the phenomenon whereby sleep helps us remember things we’ve learned during the day. And studies have borne this out. In one study, participants had to learn a motor routine (tapping buttons in a certain order). When learning the task and recalling the task were separated by a night of sleep, rather than the same amount of time during waking hours, the participants did much better. The theory is that the brain consolidates the memories it needs, but prunes back the ones it doesn’t. And research has shown that the brain tends to weaken the connections that form the memories that the brain deems unimportant.
One thing to be aware of is that sleep also seems to nail down negative memories, which probably plays a role in PTSD and depression. A very recent study showed that once negative emotional memories are consolidated during sleep, they’re less likely to be suppressed. And this means that bad memories in addition to the good ones more likely to stick around, and less likely to be forgotten.
Toxins, including those associated with Alzheimer’s disease, are cleared during sleep
One of the most illuminating discoveries in the last few years is that the brain clears out toxins much more rapidly while we’re asleep than when we’re awake.
“I think some of the most exciting work is on glymphatic system,” says Michael Thorpy, director of the Sleep-Wake Disorders Center at Montefiore Health System and professor of Clinical Neurology at Albert Einstein College of Medicine. “There’s a lot of new research in this area in last few years. The lymphatic system of the brain opens up at night, and remove toxins while we’re asleep.”
The space between brain cells expands significantly during sleep, which facilitates the clearing of the "gunk” through cerebrospinal fluid. And perhaps most astonishing is that much of this gunk is the β-amyloid protein, which is a precursor to the plaques in Alzheimer’s disease. These proteins and other toxins seem to accumulate during the day, and are cleared during sleep. Another incredibly strong reason to make sure we get enough.
Sleep is necessary for cogniti
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