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Thursday 19 December 2013

Why do we sleep? [SCIENCE VIDEO]

We spend a third of our lives doing it, yet it still isn't clear why we need to sleep. In fact, there’s so many things we don’t know about sleep, that we don’t have time to talk about them all today.

This is the first of our articles to be made available as an animated video. 
An audio version of this article is also available!

What we do know is that we can’t function without it. After missing just one night’s sleep, you are likely to find yourself feeling hungry, emotional and unable to concentrate. Decisions will become difficult to make, your reactions will slow, you may become forgetful, and your vision can even be affected. Going without enough sleep can weaken the immune system, making you susceptible to colds and other infections, and can even increase your blood pressure.

So it is clear that sleep is vital, but why? Scientists just can’t agree…
Some believe that sleep is a way to conserve energy and stay out of harm’s way[1]. Animals that stay still and quiet during part of the day are less likely to have accidents or be eaten by predators. Sleeping also uses up fewer calories than being awake, meaning animals need to find less food each day than if they don’t sleep[2].

Photograph of newborn baby yawning
Tiredness confuses scientists in more ways than one. We don't know why we yawn, either. Image credit: Bj√∂rn Rixman (Flickr/Creative Commons)

But this only really explains why we rest during the night - surely it would be safer to stay still and quiet but conscious, so if a predator stumbles upon your hiding place you have a better chance of escaping? Also, this theory might explain why sleep is a good idea, but it doesn't explain all the problems that are caused by missing out on getting enough sleep.

Other researchers think that sleep is a chance for the body to repair itself[3]. This seems quite believable - we all feel better after a good night’s sleep. However, this theory seems to suggest that people who are more physically active, so need more bodily repair, should sleep more than those who are inactive. But although there is some evidence athletes sleep more after a race than normal, those who are completely inactive don’t sleep any less[4].

However most of the impairments seen in sleep deprivation are cognitive, suggesting sleep must be important for the brain. There is some evidence that the cells in the brain responsible for repair and restoration are more active during the night[5], suggesting it may be the brain that needs time to recuperate, rather than the body.

Lab mouse at Meredith lab
Mice are often used to help us understand our own brainsImage credit: Meredith lab, VU University Amsterdam
In mice, a recent study by scientists at the University of Rochester found that cerebral spinal fluid flows around the brain 10 times faster when they are asleep[6]. This flushes out toxins more efficiently. The researchers argue that this could explain the need for sleep, and the problems that occur when we don’t get enough of it. But other researchers are sceptical, as this is yet to be shown to happen in humans[7].

Another theory is that sleep is needed for memories to be stored properly. Researchers are not clear exactly how this happens, or why sleep is needed, but studies have shown that people are better at remembering things they have learnt after a period of sleep[8]. But although this might explain the cognitive difficulties people face when sleep deprived, it can’t explain the physical symptoms.

It may be that none of these theories are correct, or that more than one of them is needed to fully explain why sleep is so important. What we do know, though, is that people who get enough sleep (around 8 hours a night for adults) are healthier, happier and more successful… what better reason to get some shut-eye?

This article was written by our Natural Sciences editor Ginny Smith. An animated version of this article is available on our YouTube channel, with animation by Es Einsteinium.

why don't all references have links?

[1] Siegel, Jerome M. Sleep viewed as a state of adaptive inactivity. Nature Reviews Neuroscience 10.10 (2009): 747-753. DOI: 10.1038/nrn2697
[2] Berger, Ralph J, and Nathan H Phillips. Energy conservation and sleep. Behavioural brain research 69.1 (1995): 65-73. DOI: 10.1016/0166-4328(95)00002-B
[3] Adam, Kirstine, and Ian Oswald. Sleep helps healing. British medical journal (Clinical research ed.) 289.6456 (1984): 1400. PMCID: PMC1443671
[4] Shapiro, CM. Sleep and the athlete. British journal of sports medicine 15.1 (1981): 51-55.
[5] Bellesi, Michele et al. Effects of sleep and wake on oligodendrocytes and their precursors. The Journal of Neuroscience 33.36 (2013): 14288-14300. DOI: 10.1523/JNEUROSCI.5102-12.2013
[6] Xie, Lulu et al. Sleep drives metabolite clearance from the adult brain. Science 342.6156 (2013): 373-377. DOI: 10.1126/science.1241224
[7] Sample, Ian. Why do we sleep? To clean our brains, say US scientists. The Guardian (18 October 2013)
[8] Fischer, Stefan et al. Motor memory consolidation in sleep shapes more effective neuronal representations. The Journal of neuroscience 25.49 (2005): 11248-11255. DOI:10.1523/JNEUROSCI.1743-05.2005

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