Source: BBC

There are a few things we literally cannot live without. Oxygen is one. Another is food and water. And then there is sleep:forcibly keep an animal awake for long enough and you will kill it. The same almost certainly applies to humans.

This fact alone tells us that sleep must be doing something pretty important. But despite decades of intense scientific study there still is no consensus on exactly what that something is.

Researchers have found that sleep is beneficial to humans in many ways: it helps us process memories, and keeps our social and emotional lives on track. Yet we still do not really know how, why or even exactly when sleep evolved.

A white rhinoceros (Ceratotherium simum) takes a nap (Credit: Ulrich Doering/Alamy Stock Photo)

At first glance, sleep really should not exist. It makes very little sense for animals to deliberately lose consciousness, sometimes for hours on end every single day.

A sleeping animal is significantly more likely to be caught and eaten than a waking animal

“The cost of losing consciousness to survival is astronomical,” saysMatthew Walker at the University of California in Berkeley. Whatever functions sleep performs, they must be so fundamentally important that they far outweigh the obvious vulnerability associated with being asleep.

This means we can confidently reject one of the simplest theories of sleep: that we drift off simply because we have nothing better to do.

This could be described as the indolence theory of sleep. Once an animal has eaten, seen off any rivals and exhausted any potential mating opportunities, it effectively has an empty schedule. With no urgent business to attend to, losing consciousness kills time for a few hours.

It is a fun idea, but considering that a sleeping animal is significantly more likely to be caught and eaten than a waking animal, this hypothesis makes “zero sense”, says Walker.

We might as well cross another theory of sleep off the list, too.

Staying awake can keep you safe (Credit: Lauren Pretorius/Alamy Stock Photo)

Some researchers have suggested that sleep is a good way to save valuable energy, given that mammalian core body temperatures often drop during some stages of sleep.

Does a housefly sleep? How about an earthworm?

However, Walker and many other sleep researchers are not convinced. Crunching the numbers makes it clear that sleep does not serve this purpose.

“The amount of energy humans save by falling asleep, versus simply lying on the couch, is about what you find in a slice of brown bread,” he says. “Losing consciousness just isn’t worth saving 120 calories.”

But what is sleep for, if these ideas are out? Before we can really think about answering that question, it would be helpful to address something more fundamental: who sleeps?

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