The excellent web-toon XKCD said it best when it pointed out that we all spend so much time sleeping—and most of us love doing it—but we still don’t know why. But never fear, some scientists finally think they may have a clue—a discovery so significant it was a runner-up for Science magazine’s Breakthrough of the Year.
(The quote in the alt text—and you should always read XKCD’s alt text—is from a National Geographic story where they interviewed Stanford University’s William Dement, a co-discoverer of both REM sleep and narcolepsy, who said, after 50 years of sleep research, “As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy.”)
Of course, we do know that we need sleep, on the basis that insomnia and sleep deprivation are harmful. The trouble is that how exactly they’re harmful isn’t known either—experiments by Allan Rechtschaffen in the 1980s found that sleep deprivation was eventually fatal to rats, but a specific cause of death couldn’t be found.
Naturally, there are theories:
- An obvious one would be that sleep conserves energy, which is fine except that we use only 5-10% less energy asleep than awake, which from an evolutionary point of view doesn’t sound enough to justify the increased vulnerability to predators.
- However, it could also help protect from predators, as hiding somewhere without moving is safer than running around in the open; although again, simply sitting still, or quiescence, would be just as effective and make it possible to react in an emergency.
- Growth hormones do seem to be influenced by sleep, but there doesn’t seem to be a strong correlation between children’s growth and their amount of sleep; plus, it doesn’t explain why adults sleep.
- Memory does benefit, as numerous studies have shown that a good night’s sleep helps you remember what you learned the previous day, with some suggesting that it gives the brain the chance to remove unnecessary connections. Maybe not the whole story though, as animals with very simple brains also go through a sleep cycle.
- Restoring the body may be closer to the mark, as a 2004 study (also on rats) showed that sleep deprivation may slow the healing of wounds.
- The immune system also seems to benefit, as further rat studies found that sleep deprivation reduced white blood cell counts and increased other cells and chemicals that encourage cancer growth.
Furthering this theme of restoration, a new study by researchers led by Maiken Nedergaard at the University of Rochester in upstate New York, has found a connection between sleep and a brain-cleaning system they discovered (Xie L, Kang H, Xu Q, Chen MJ, Liao Y, Thiyagarajan M, O’Donnell J, Christensen DJ, Nicholson C, Iliff JJ, Takano T, Deane R & Nedergaard M 2013, “Sleep drives metabolite clearance from the adult brain”, Science, vol. 342, no. 6156, pp. 373–377, DOI: 10.1126/science.1241224 [PDF 1.9 MB]).
They call this the “glymphatic system”. It’s similar to the lymphatic system, and it actually connects to it. Lymph comes from the interstitial fluid, the stuff between cells, which has its own circulation system that removes bacteria and toxins, sending them through the lymph nodes and eventually to the veins so they can be cleaned from the body.
However, the lymphatic system doesn’t go through the brain. That’s because the brain has its own cerebro-spinal fluid. This is the liquid that the brain sits in—it actually floats in a big bag of the stuff, which prevents it being damaged by its own weight, as well as cushioning it against injury.
What Nedergaard’s team discovered is that this cerebra-spinal fluid (CSF) also flows through channels around ordinary blood vessels, and then on through other, smaller conduits formed by glial cells (these are cells in the nervous system that aren’t neurons—the name actually comes from the Greek word for “glue”, in that they glue the nerves together. Basically, they’re the support system for the nervous system).
This flow of the CSF removes waste products that neurons produce, things like like beta amyloid, which is a protein that accumulates and forms sticky plaques in patients with Alzheimer’s disease.
For such an important system, it’s surprising that it was only identified a couple of years ago. But that’s until you consider that it only operates in living brains: it needed to wait for the development of sophisticated brain scanning technology.
In this case, they used a technique called two-photon microscopy, in which fluorescent dyes are activated by two photons in the infrared range, which penetrates further into tissue.
With this method they found that during sleep the brain cells reduce in size by 60%, creating more space between them for the cerebro-spinal fluid to flow through and flush out waste products.
This is consistent with other research that shows that levels of beta amyloid declines in human brains during sleep, although they haven’t confirmed that it does the same in mice. However, as Nedergaard points out, “Isn’t it interesting that Alzheimer’s and all other diseases associated with dementia, they are linked to sleep disorders?”
Whether this is still the main explanation for why we sleep is still unknown, but as other experts have said, it does demonstrate one clear physiological function. And it’s consistent with the other studies that suggest restorative benefits, like with wounds and the immune system.
So another reason to get a good night’s sleep, to dream pure thoughts and wake up with a fresh mind. Not like you needed me to tell you that…
(This story first aired on 10 April 2014—you can listen to the podcast.)