Exploring the function of sleep

Is sleep essential? Ask that question to a sleep-deprived new parent or a student who has just pulled an "all-nighter," and the answer will be a grouchy, "Of course!"

But to a sleep scientist, the question of what constitutes sleep is so complex that scientists are still trying to define the essential function of something we do every night. A study published this week in PLoS Biology by Chiara Cirelli and Giulio Tononi addresses this pressing question.

The search for the core function of sleep can seem as elusive as the search for the mythological phoenix, says Cirelli, an associate professor of psychiatry at the University of Wisconsin School of Medicine and Public Health in Madison.

Some scientists argue that sleep is merely a way to impose a quiet, immobile state (rest), and isn't important by itself in mammals and birds. This is the so-called "null hypothesis," and Cirelli and Tononi reject it.

"We don't understand the purpose of sleep, but it must be important because all animals do it," Cirelli says.

There's no clear evidence of an animal species that doesn't sleep, she says. Even the dolphin—which is sometimes held up as an animal that doesn't sleep because it moves continuously—will show "unihemispheric sleep" with one eye closed and one half its brain showing the slow waves characteristic of deep sleep.

"The very fact that dolphins have developed the remarkable specialization . . ., rather than merely getting rid of sleep altogether, should count as evidence that sleep must serve some essential function and cannot be eliminated," Cirelli says.

She also argues that sleep is strictly regulated by the brain, because sleep deprivation is followed by a rebound, in which the sleep-deprived animal either sleeps longer, or spends more time in the deeper sleep characterized by large slow brain waves.

Prolonged sleep deprivation has been shown to kill rats, flies and cockroaches. Humans who have a genetic insomnia can also die. In less extreme cases, sleep deprivation affects cognitive function in animals ranging from flies to rodents. Rats kept awake will engage in "micro-sleep" episodes, and sleep-deprived humans tend to fall asleep even in the most dangerous circumstances.

Because it is universal, tightly regulated, and cannot be lost without serious harm, Cirelli argues that sleep must have an important core function. But what?

"Sleep may be the price you pay so your brain can be plastic the next day," Cirelli and Tononi say.

Their hypothesis is that sleep allows the brain to regroup after a hard day of learning by giving the synapses, which increase in strength during the day, a chance to damp down to baseline levels. This is important because the brain uses up to 80 percent of its energy to sustain synaptic activity.

Sleep may also be important for consolidating new memories, and to allow the brain to "forget" the random, unimportant impressions of the day, so there is room for more learning the next day. This could be why the brain waves are so active during certain periods of sleep.

"While there may still be no consensus on why animals need to sleep, it would seem that searching for a core function of sleep, particularly at the cellular level, is still a worthwhile exercise," she concludes.

Citation:Cirelli C, Tononi G (2008) Is sleep essential? PLoS Biol 6(8): e216. doi:10.1371/journal.pbio.0060216 biology.plosjournals.org/perls … journal.pbio.0060216

Source: Public Library of Science

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Aug 26, 2008
I personally believe sleep is the process of the brain transferring information from short term memory to long term.

Sometimes after really deep sleep, I have to spend a few seconds reloading my short term memory with where I am, whats going on in my life currently, etc.

Aug 26, 2008
Sleep may also be an example of exaption. If a species developed a behaviour of shutting down for part of the day (for instance decreased activity during the day while all the big animals are about), then evolution might take advantage of that to add some "features". For instance, that would be a good time to step up healing processes. Therefore, the animal evolves to heal less while active and more while inactive. Then the animal evolves to do more "synaptic cleanup" while inactive. Each added feature would make the species more dependant on sleep, but would also give it an advantage while active. Eventually sleep would become a core part of the species' behaviour because so many different processes would depend on it.

Aug 26, 2008
@ d666: But then it seems that at least one species would have weeded out sleep entirely. If evolution can give rise to a platypus, surely one animal would have opted out of sleep, especially with it's obvious down side.


Aug 26, 2008
ancible: that's true if sleep developed later on. But if sleep developed extremely early in the evolutionary process then it is quite possible that all current animals are descendants of the species that first developed sleep (and thus gained an advantage). Kinda like mitochondria. Are there any multicellular animals (or plants too, I think?) that don't have mitochondria? Yet it is possible, because there are a lot of single celled organisms that don't have them. It just so happens that here, on Earth, evolution ended up going down this one path. That doesn't mean that it is the *only* path that it could have gone down, or even the best path.

I agree with D666.

Aug 27, 2008
Based on effects of sleep deprivation on cognitive abilities I believe sleep is required for sustained neural communication.

The longer you stay awake beyond what would be a normal day length the more you get the feeling of parts of your brain disconnecting from the consciousness loop, and each basic mental task gets progressively harder until you can only perform the most basic operations.

The easiest explanation would be that in order to increase their processing power (and competitive advantage of the animal) nerve cells use up more neurotransmitters during the day then they are able to produce, then during the night they replenish reserves.

This hypothesis would explain some other paradoxes of brain function, for example certain substances like amphetamines can increase your cognitive capacity and energy (and this increase has been objectively confirmed).
The substance is just a trigger which means that you always have the potential to perform better its just not normally accessible. But why isn't it accessible if it offers a clear advantage?

The answer is that the drug allows your brain to use neurotransmitter reserves faster and in that way increases your capacity, but you pay the price for it - when the reserves finally run out you are left disoriented and much more vulnerable. In other words you get the comedown.

From evolutionary point of view it is more important for animal brains to perform consistently so that when the predator attacks they are ready. Thats why evolution tweaked our brains to use no more neurotransmitters during the day then we are able to replenish during the night and to always have something like a 2 or 3 day reserve.

The key factor in my hypothesis is that there is something that is needed for nerve cells to communicate efficiently that is used up faster than it can be replenished during normal brain activity. The first guess is that it is neurotransmitters, but it could just as well be some other element of signaling cascade involved in neural communication, some other molecule, protein complex, or cellular component.

Aug 27, 2008
@gopher: I see where you're coming from. But even D666 said:

"Then the animal evolves to do more "synaptic cleanup" while inactive. Each added feature would make the species more dependant on sleep, but would also give it an advantage while active. *Eventually* (emphasis ancible's) sleep would become a core part..."

My feeling is that before sleep became needed a very early animal would have gone down a different, sleepless path. However, if there is some fundamental burnout to neuron's due to their very design (as superhuman said) then we would see all animals with a brain complexity of n or more needing sleep. Which is what we see (so far).

Aug 28, 2008
Here's a very fundamental question, to which I do not know the answer: Do very very simple mulicellular organisms sleep? How about worms? Ants? I'm fairly sure that bacterial don't sleep, but what about animals that barely have nervous systems?

BTW, I like gopher's point about mitochondria. A similar point could be made about chloroplasts. It is possible in principle for organisms to independantly evolve alternatives to these, but once an organism *did* evolve the first solution, it would have such a huge advantage that there probably wouldn't be any chance of competition evolving.

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