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Eat, drink, breathe, sleep. These are probably the four most basic things a person needs to live. But while it seems obvious why we would need to eat, drink, and breathe, the purpose of sleep is a little less clear. We need oxygen, fuel, and water to power our bodies—but why can’t we just keep going without dozing off every 16 hours or so? What do these periodic breaks from waking life provide? Why do we sleep?

We ask this question because sleep is obviously important. We spend a third of our lives doing it. If we don’t get enough, it can ruin our mood, or even kill us. Countless religions have had a god of sleep, and sleep has formed the basis of our stories from Sleeping Beauty to Rip Van Winkle. Nobody has to be convinced that sleep matters for humans to function…but why?

Why does a lack of sleep make a person psychotic? Why will it eventually kill you? Well, despite the fact that people have been studying sleep for centuries, and that it continues to be the source of intense research today, we don’t fully understand the purpose of sleep—but we’re starting to get an idea. It turns out that there’s far more going on than meets the eye when we doze off each night.

Animal Instinct

First of all, it’s key to know just how essential sleep is—not just to us humans, but to nearly every single form of animal life on earth. Apart from a very select few creatures that have either no brain or an extremely rudimentary one, every single animal that we’ve observed sleeps in some way. That’s from 190-ton blue whales to near-microscopic worms. Even sharks, who need to keep moving constantly or else they’ll suffocate from a lack of oxygen, manage to do it. Sleep, it would seem, is a vital part of almost every creature’s life—despite the fact that it seems like a really bad idea, evolutionarily speaking

We take sleep for granted so much that most people don’t stop to think about how odd of an activity it is. After all, if the goal of life is to survive and reproduce, you’d think sleep was a bad idea—you’re much more likely to get eaten up by a predator if you’re asleep and can’t see it coming. But even with the risks involved, animal life has always needed to sleep. Clearly, the advantages of sleep outweigh the disadvantages—so what are those advantages? Though the answer may change from animal to animal, at least as far as humans go, science has come up with several theories.

Saving Up

One of the earliest theories into the purpose of sleep posits that we sleep in order to conserve energy. Brains use an extraordinary amount of energy, especially in humans—up to 20% of our total consumption. Since the brain uses far less energy while asleep, it could be that these savings gave our ancestors enough juice to find the food they needed to survive.

This theory has been around for many years, and it’s undeniable that our bodies use far less energy while sleeping, but it probably doesn’t fully explain the phenomenon. For one, it completely ignores the restorative properties of sleep, which leads us to another popular theory.

Time to Repair

Repair and restoration is another function that’s believed to be an integral part of why we sleep. Sleep allows our bodies to repair themselves and remove metabolic wastes that can build up during the waking hours. This regular maintenance seems to play an extremely important role in maintaining our immune systems. In several animal tests, it’s been found that sleep deprivation can lead to the complete shutdown of all immune functions.

It seems fairly clear to scientists that repair and restoration are at least part of why we sleep, but yet again, it’s likely not the sole reason. As scientists have done more and more research, signs have begun pointing to something called “brain plasticity” as another of sleep’s key functions.

Paper or Plastic

In simple terms, plasticity refers to your brain’s ability to form new connections between neurons. The brain essentially reorganizes itself constantly, and this is how we’re able to learn new things and form new memories. Plasticity is also how the brain is able to repair itself after it’s damaged.

The link between brain plasticity and sleep is becoming clearer with each passing year. Studies have shown that a lack of sleep seriously affects our ability to learn and memorize new things. It’s also been shown that sleep is critical for an infant’s brain development—partially explaining why babies sleep so much.

Deeper Questions

If you were to ask a research scientist why we sleep, they might give you one of these answers. Each time we drift off, our bodies conserve energy, repair and refresh themselves, and our brains do some regular maintenance that helps with learning and memory. Sleep also helps our immune and endocrine systems function and maintains our mood and cognitive abilities. But does that really answer the question, “Why do we sleep?” Well, that’s up for debate.

Since nearly every single animal that has a brain has been observed to sleep, it would seem that sleep evolved very, very early in the evolutionary tree. As in hundreds of millions of years ago, back when brains were a lot simpler than ours. For that reason, sleep looks very different across the spectrum of life. Not every animal gets all the same benefits from sleep as a human—and yet nearly every animal still does it. A near-microscopic worm with only 302 neurons sleeps. Even neuron networks that scientists have grown in labs sleep. This has led several scientists to believe that sleep even more important that we ever thought—that you can’t even have a brain without it.

So while we know of several key functions of human sleep that can help to answer the question “Why do we sleep?”, it may go much, much deeper than that. Not every animal gets the same energy conservation, restoration, and brain plasticity benefits as humans do, but they sleep all the same. Now, as for the question: “Why does every single brain, even the most simplistic, sleep?” That’s perhaps the most interesting question of all. It’s just not one we have an answer for.

Sources:

1, 2, 3, 45

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