Cirelli, Tononi. “Is Sleep Essential?” PLoS Biology Essays, 6(8) 2008.
HELL YES SLEEP IS ESSENTIAL. I say that right now because I have had two pots of coffee already today. I am still sleepy, but now also jittery. So anytime anyone makes a loud noise I jump and my heart races, and then I fall back into a half sleep. Sleep is good for you, folks.
But apparently there is somewhat of a controversy in science as to whether or not sleep is essential. Or rather, not whether sleep is essential, but whether it evolved because it serves a function in itself. There are two sides to the debate. One side says that sleep probably serves a special function in itself. The other side says that sleep is something that mostly happened to keep you from moving around and wasting energy when you don’t need to (think hibernation), and that THEN other functions came to go with sleep, and now sleep, though it doesn’t have a function in itself, is important because of the other things that happen during it.
The null hypothesis of this essay is as follows: Sleep is just one out of a repertoire of behaviors, and is useful for certain things, but is not essential. This is not to say that you don’t NEED sleep, because of course we all do (heck, I want to go to bed now, and it’s only 8:30!) The idea is that sleep evolved as just one of many things you could do, and was sometimes the best option in the circumstances. Over time, sleep took on different roles and became useful for certain things. This would go a good way to explaining why sleep times vary so much between species.
If it is true that sleep is not an essential behavior to animals, the paper states that three things would have to be the case:
1) There would be animals that don’t sleep at all
2) There would be animals that don’t need recovery sleep when they stay awake too long
3) Lack of sleep would occur without serious consequences.
But first, what IS sleep? Sleep is defined as “a reversible condition of reduced responsiveness usually associated with immobility”. Basically, you’re not as responsive if someone tries to get your attention, but it’s reversible if you need to wake up for something important. Immobility is not necessarily required (sharks, for instance, have to keep swimming while sleeping to keep water flowing over their gills). Sleep is also often defined as changes in your brain. During non-rapid eye movement sleep, your brain goes into these slow firing waves that are characteristic of a sleep state in many species. Unfortunately, these waves are hard to define in reptiles, fish, or invertebrates (who may not be capable of such waves at all).
The first question is, are there animals that don’t sleep? I don’t know about you, but I wish I was an animal that didn’t sleep! I’d get so much more done! Sleep has actually only been studied extensively in a few animal models, most of them models that are used extensively for other kinds of studies. It’s pretty well established that all mammals sleep. Sleep has also been found in fruit flies, zebrafish, and even C. elegans may show a sleep-like state (though I imagine it’s very hard to tuck a nematode in at night and sing it lullabies).
Where sleep has been difficult to track has been in animals like bullfrogs, which don’t appear to be less responsive even when they have been quietly resting for some time. But it appears that sleep is very important to mammals. Dolphins have even evolved a way to sleep while swimming. They have unihemispheric sleep, where half the brain goes in to sleep waves, and the other half does not. Because one half of the brain controls the other half of the body, shutting half the brain down means that the dolphin will swim in lazy circles. When the halves switch, I imagine the dolphin switches, too.
Can sleep loss happen without compensation? We’ve all done it. Stay up late one night (or stay up all night), and the next day you’re a zombie by 3 pm, you pass out and sleep 14 hours straight, and wake up wondering what happened. Sometimes, with chronic loss of sleep, you make up for lost time by sleeping deeper (with more slow waves), and waking up less often in the night. But it appears that not all animals needs to make up for lost sleep. Pigeons appear to have no changes in total sleep time, even when they’ve been kept awake for almost 10 days! However, they did find evidence that slow waves were creeping into the birds’ daytime brain activity. Does that make for narcoleptic birds? In this essay, the authors conclude that sleep is strictly regulated in all species studied so far, but I think more work might need to be done here.
Can sleep loss occur without problems? Problems after sleep loss have been very clearly documented in humans. Chronic insomnia can lead to narcolepsy, and there is a very severe case of fatal familial insomnia, which actually leads to death. The syndrome is a fatal inherited prion disease, which starts with insomnia, anxiety and panic attacks, progresses to hallucinations and dementia, and ultimately results in death, though whether or not they actually die from lack of sleep is still up for debate. Sleep loss leading to death has also been shown in rats, flies, and cockroaches.
Chronic sleep leading to narcolepsy is actually called “sleep intrusion” (real narcolepsy is something quite different). Rats being kept awake all the time will still get about 10% of their sleep due to “microsleeps”. In humans, a very motivated person (like Randy Gardner) can stay awake for up to 11 days, but seriously sleep deprived people will fall asleep no matter where they are. And lack of sleep isn’t good for us. Partial lack of sleep can leave to cognitive impairments in humans (get your 8 hours, folks!), and this has also been shown in monkeys.
Are you sleepy, or are you tired? Being “sleepy” is more of a function of your biological clock. For instance, when you have jet lag, you’ll get sleepy at the wrong times of day until your clock adjusts, and this will happen even though you slept a good amount of time and don’t NEED the sleep. But are your brain and body tired rather than sleepy? Do we need that time asleep to do some tune-ups around the house? Some PET studies have shown that glucose metablism (use of energy) in certain areas of the brain is decreased when you’re tired, which implies that sleep may be needed for upkeep of some brain areas.
So is sleep essential? The authors here appear to posit that it is, and that the real function of sleep simply hasn’t been found yet. Presumably it would be an important function. Fish asleep in reefs are much more likely to be eaten, and it’s hard to think of things that are more important than not being something else’s late night snack. However, it is possible that sleep was not as essential as it is now. It is possible that we have evolved functions, such as memory consolidation and turnover of cells in the gut, that take advantage of the lack of activity during sleep. Enough of these late-night functions could have made sleep go from a mere behavioral option to something far more essential. It may not have been essential before, but it’s pretty clear we need it now.
Cirelli, C., Tononi, G. (2008). Is Sleep Essential?. PLoS Biology, 6(8), e216. DOI: 10.1371/journal.pbio.0060216
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