Why do we sleep?

Feb 03, 2011 By Marcus Woo
Credit: Chau Dang, LTD Space

While we can more or less abstain from some basic biological urges—for food, drink, and sex—we can’t do the same for sleep. At some point, no matter how much espresso we drink, we just crash. And every animal that’s been studied, from the fruit fly to the frog, also exhibits some sort of sleep-like behavior. (Paul Sternberg, Morgan Professor of Biology, was one of the first to show that even a millimeter-long worm called a nematode falls into some sort of somnolent state.) But why do we—and the rest of the animal kingdom—sleep in the first place?

“We spend so much of our time sleeping that it must be doing something important,” says David Prober, assistant professor of biology and an expert on how genes and neurons regulate . Yes, we snooze in order to rest and recuperate, but what that means at the molecular, genetic, or even cellular level remains a mystery. “Saying that we sleep because we’re tired is like saying we eat because we’re hungry,” Prober says. “That doesn’t explain why it’s better to eat some foods rather than others and what those different kinds of foods do for us.”

No one knows exactly why we slumber, Prober says, but there are four main hypotheses. The first is that sleeping allows the body to repair cells damaged by metabolic byproducts called free radicals. The production of these highly reactive substances increases during the day, when metabolism is faster. Indeed, scientists have found that the expression of genes involved in fixing cells gets kicked up a notch during sleep. This hypothesis is consistent with the fact that smaller animals, which tend to have higher metabolic rates (and therefore produce more free radicals), tend to sleep more. For example, some mice sleep for 20 hours a day, while giraffes and elephants only need two- to three-hour power naps.

Another idea is that sleep helps replenish fuel, which is burned while awake. One possible fuel is ATP, the all-purpose energy-carrying molecule, which creates an end product called adenosine when burned. So when ATP is low, adenosine is high, which tells the body that it’s time to sleep. While a postdoc at Harvard, Prober helped lead some experiments in which zebrafish were given drugs that prevented adenosine from latching onto receptor molecules, causing the fish to sleep less. But when given drugs with the opposite effect, they slept more. He has since expanded on these studies at Caltech.

Sleep might also be a time for your brain to do a little housekeeping. As you learn and absorb information throughout the day, you’re constantly generating new synapses, the junctions between neurons through which brain signals travel. But your skull has limited space, so bedtime might be when superfluous synapses are cleaned out.

And finally, during your daily slumber, your brain might be replaying the events of the day, reinforcing memory and learning. Thanos Siapas, associate professor of computation and neural systems, is one of several scientists who have done experiments that suggest this explanation for sleep. He and his colleagues looked at the brain activity of rats while the rodents ran through a maze and then again while they slept. The patterns were similar, suggesting the rats were reliving their day while asleep.

Of course, the real reason for sleep could be any combination of these four ideas, Prober says. Or perhaps only one of these hypotheses might have been true in the evolutionary past, but as organisms evolved, they developed additional uses for sleep.

Researchers in Prober’s lab look for the genetic and neural systems that affect zebrafish sleeping patterns by tweaking their genes and watching them doze off. An overhead camera records hundreds of tiny zebrafish larvae as they swim in an array of shallow square dishes. A computer automatically determines whether the fish are awake or not based on whether they’re moving or still, and whether they respond to various stimuli. Prober has identified about 500 drugs that affect their sleeping patterns, and now his lab is searching for the relevant genetic pathways. By studying the fish, the researchers hope to better understand sleep in more complex organisms like humans. “Even if we find only a few new genes, that’ll really open up the field,” he says. The future is promising, he adds, and for that, it’ll be well worth staying awake. 

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antialias
4.8 / 5 (4) Feb 03, 2011
Someone once asked the question the other way around: why are we even awake? Of the two states being asleep is more efficient. We only wake out of necessity (food, procreation, security)

Many animals optimize for duration of sleep (e.g. any species that hibernates)
Moebius
3 / 5 (2) Feb 03, 2011
The first one is wrong, sleep isn't necessary for repair of body cells. The brain is deeply involved with sleep. It is probably mostly the second, replenishing fuel, and some of the third and fourth too.

If I sleep too much my head hurts and I feel bad, can't force myself to sleep more, much as I would like to. That sounds like the ATP has a negative effect if too much is trying to be created from too much sleep, a hypothesis that should be able to be tested.
HyperAnomaly
5 / 5 (2) Feb 03, 2011
I'm not sure about the quality of this article. ATP concentration is more or less kept constant at plus or minus 10% or you will die, and each molecule is recycled >1000 times a day. If I recall correctly, the metabolic control is at AMP. Perhaps the author was confused?
pauljpease
3.8 / 5 (4) Feb 03, 2011
I disagree that there are only four possible reasons for sleep. Why didn't they mention the simplest hypothesis, which is based firmly on evolutionary theory? Evolution predicts that traits that help us survive and reproduce should become predominant in the population. So, in what ways might sleeping for some fraction of the day improve our chances of surviving to reproduce? The less time we are awake, the less chance of having an accident, so sleeping reduces our chance of dying by accident. In other words, why risk dangerous waking activities if they are unnecessary? If this is true, then we should be optimized for getting enough food, water, shelter and sex in as few hours as possible, so we can be idle/sleeping the rest of the time. A test of this hypothesis would be to check if people sleep less when their basic needs aren't met. When needs aren't met there is usually stress/anxiety, and obviously people who are anxious/stressed tend to have a harder time sleeping.
pauljpease
3.3 / 5 (3) Feb 03, 2011
I would argue that sleep evolved as a way of minimizing risks. You can only eat and reproduce so much in a given period of time, so evolution has selected for traits that allow organisms to get as much food and sex as possible in a given period, and then idle for the rest of the time. It does not increase your chance of survival or reproduction to be gathering food if you're already full, for example.

Once the behavior of sleep arose, initially to conserve energy and minimize risk, then other changes could happen to take advantage of this idle time. For example, brains would be able to use that down time for learning. As they said in the article, sleeping brains run simulations of waking activities. In other words, sleep became a great way to learn by experiences, but in a virtual reality that has no real-world risks. So sleeping some fraction of the day keeps you safer on average, and dreaming while you sleep allows your brain to make use of that down time to continue learning.
pauljpease
3 / 5 (2) Feb 03, 2011
By the way, anything that has a chance of breaking while in operation tends to have "sleep" behavior. For example, it would be silly if I left my car running when I'm not using it. I actually think that if a car were a product of biological natural selection, they would only bother burning gas to either get to a gas station, get to a mechanic, or carry auto workers to a factory so they can build more cars. Since it can't do those things 24/7, it stands to reason that it would be beneficial to spend a lot of time resting.
mysticshakra
1 / 5 (3) Feb 05, 2011
Another modern failure for.science. Another.thing it doesnt know and cant explain. No doubt the arrogance will nevertheless continue unbated however. I find it amusing that science has the most difficult time explaining the simple things that we all take part in.
Moebius
3 / 5 (2) Feb 05, 2011
Another modern failure for.science. Another.thing it doesnt know and cant explain. No doubt the arrogance will nevertheless continue unbated however. I find it amusing that science has the most difficult time explaining the simple things that we all take part in.


More religious crap. This same line has been used many times over the years and promptly falls apart when science DOES find the missing explanation, as it will with sleep.