For decades, scientists have been trying to figure out why, exactly, humans need to sleep so much. And after all this time, no one knows for sure. But there have been a variety of interesting hypotheses proposed.
Some researchers think that sleep helps restore our bodies. Others have suggested that it clears out toxins from our brains. Still others have suggested we evolved to sleep so much in order to hide from predators.
These hypotheses aren't mutually exclusive. It's entirely possible that many of them are true and that over time, sleep has come to provide many functions. But why we sleep is a crucial question — figuring it out might someday help us understand why it's so important to get enough (or, alternatively, help us develop drugs that let us go without).
For now, here are five of the most interesting ideas about why humans sleep:
1) Sleep clears our brains of junk
This is a very new area of research, but recent studies in mice have shown that the brain seems to prefer to rid itself of trash at night. It's possible that without sleep, this toxin buildup helps cause some of the nasty effects of sleep deprivation.
In 2012, researchers from the University of Rochester first described the brain's drainage system, called the glymphatic system (named after brain cells called glia and the lymphatic system). This is a series of watery channels that wash junk and toxins out of the brain.
Then, in October 2013, the researchers showed that the cleaning seemed to be happening primarily while the mice were asleep. This was an experiment that involved training mice to sleep under a microscope and injecting them with fluorescent dye to watch the liquid flow.
Intriguingly, the researchers found that when they injected the mice with beta-amyloid protein, whose build-up causes Alzheimer's disease, it was washed out twice as fast during sleep as during waking. This is especially interesting because poor sleep is a risk factor for Alzheimer's.
2) Sleep strengthens our memories
Another hypothesis is that sleep helps us consolidate memories.
There's lots of evidence to support this. In some studies, scientists give people something to remember and then had them sleep in a laboratory. (The subjects are hooked up to an EEG device so that researchers can see what phase of sleep they're in.) The researchers would then wake them up at various stages and test their memories the next day.
Over time, they've been finding that all kinds of sleep stages seem to be associated with strengthening memories, as well as discarding less important information. That helps us remember not just facts and emotions, but also things like how to perform a new physical task.
(Initially, scientists thought that REM sleep was most crucial for memory strengthening, but there's been some recent evidence against this. For example, some people with brain injuries are unable to get REM sleep, but they can still remember things.)
Meanwhile, other studies have found that rodents seemed to replay brain patterns during sleep that are associated with experiences while awake. That suggests that the animals are strengthening the synapses involved in these memories.
So why couldn't you just do this while awake? There are a number of ideas about that, too. One major hypothesis is that the brain needs to do these tasks without interference. The synaptic homeostasis hypothesis posits that the connections between neurons need to pretty much reset themselves every night.
3) Sleep restores our bodies
The brain isn't the only thing that sleeps. The whole body is involved — lying mostly still for hours and hours on end. And some sleep researchers hypothesize that resting and repairing the body is a major reason for sleep, although it's unlikely to be the only one.
Evidence shows that deep sleep seems to be the preferred time for the body to release growth hormone and make proteins associated with tissue repair. The advantage to doing this during sleep is still being investigated.
Another interesting possibility here is that sleep somehow allows the immune system to function better. For example, rats deprived of sleep end up with sores on their bodies (which might be related to the immune system) and die within weeks. And in an experiment, people who regularly slept less than seven hours a night were about three times more likely to get sick when exposed to the common-cold virus than people who slept eight hours or more. And overall, species that tend to sleep more have more immune cells and fewer parasites than those that sleep less.
4) Sleep conserves energy
This one makes more sense if you think about our evolutionary ancestors struggling to find enough to eat. Sleeping bodies use up less energy than ones that are awake, which means that sleep could be a way to ramp down energy consumption and help us make the most of limited food.
There's some plausible evidence for this: both body temperature and metabolism decrease while sleeping. (Metabolism can drop 10 percent in people.) And rats deprived of sleep end up having trouble regulating their body temperature.
This hypothesis might help explain why many creatures sleep at night rather than during the day. Night is colder than day — so warm-blooded creatures would have to expend even more energy to stay warm if they were awake. Better, then, to go into somewhat-stasis mode and save energy up for only the few hours of eating, mating, and other activities required for survival.
5) Sleep helped us hide from predators
Some researchers have suggested that we (and other animals) might have evolved to sleep during the night in order to evade predators. After all, sleep allows us to hide, be quiet, and be still for a long time.
However, there's one obvious potential problem with this hypothesis. In many ways, sleeping animals are the perfect prey. They're vulnerable. They generally have their eyes closed and, depending on the species, are less sensitive to noise, touch, and smells — everything that could help them escape if a predator were nearby.
Indeed, some researchers have argued that the vulnerability of a sleeping animal is reason to believe that sleep must be doing some (other) very important things. Otherwise, evolution would have destroyed this reckless behavior eons ago.