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What is sleep, even?

four sleeping children.
Rest up.
Katherine Ellen Foley
By Katherine Ellen Foley

Health and science reporter

This article is more than 2 years old.

There’s a good chance that when you woke up this morning, your first thought was to go back to bed. But it’s also likely that you had so many obligations pulling you out from under the covers—work to do, food to cook, friends to see—that you started your day, even if you didn’t get your seven to eight hours of recommended shuteye.

It’s common to try to cram more waking hours into each day. About half of people worldwide get less than six hours a night, according to the Guardian.

To a certain extent, it makes sense that we’d try to cut corners on this aspect of our biological maintenance. From the outside, sleep seems like a huge timesuck. Allan Rechtschaffen, formerly a professor at University of Chicago, wrote in 1998 (paywall) that while we sleep “we do not procreate, protect or nurture the young, gather food, earn money, write papers, et cetera.” We are off our sensory grid, unaware of our surroundings and inattentive to our responsibilities.

But more time awake doesn’t mean we get more done: According to a study by the RAND Corporation, the US, Canada, UK, Germany, and Japan lost a total of $680 billion each year due to sleep deprivation (technically, anything less than seven hours a night for adults).

That might be because, while the rest of our bodies are unmoving, our brain exhibits a whirlwind of maintenance activity.

During our waking hours, we’re constantly taking in new information. As we do, our neurons make connections among themselves called synapses. During sleep, those same neurons examine the synapses made during the previous day or (or two, or three, if you’ve been learning something over time). Collectively, the brain “samples them [to] assess their overall strength,” says Chiara Cirelli, a psychiatrist at the University of Wisconsin-Madison. Then, it decides what’s vital and what’s not.

The brain, therefore, is not at rest even if you are. “All your brain is active,” Cirelli says. “Neurons are firing, they’re just firing in a different way.” Earlier this year, Cirelli and her team published (paywall) work analyzing images of mouse brains showing that that overnight the brain cuts back on some of the connections it’s made during the day. Rather than building up connections as it does during the day, at night it’s working to cut them down to only the ones you’ll need later. The synapses that involve memories that matter, like material you studied for a test, or a conversation with a loved one, are kept and sometimes even strengthened. The synapses involving memories that don’t, like what you had for lunch, are lost.

It would be unethical to conduct the invasive brain surgery needed to study this exact function on healthy human adults, but Cirelli is confident a process similar to what she found in mice happens in all types of animal brains. Strong synapses take up a lot of caloric energy to maintain, and even physical space, she says. If we kept every synaptic connection we made during waking hours, our brains eventually wouldn’t fit in our skulls.

This neurological housekeeping is essential, but there are other benefits to sleep. It provides our muscles and organs a chance to rest, for example. In theory, we could get this from any time physically relaxing, but realistically sleeping through the night is the only way most people will fully slow down.

There might be benefits of sleep that we don’t yet understand, too. Scientists can only study sleep in humans by keeping willing participants awake for longer than they’re comfortable—which means, to date, sleep studies have been relatively limited. From those limited studies, it’s clear there are real health consequences of not sleeping, including hypertension, diabetes, and mood disorders. These studies, however, only look at mild sleep deprivation; pushing a human beyond a couple of days of sleeplessness is literally and legally torture. Total sleep deprivation has been applied to rats, and studies suggest 11 days-worth would kill us (paywall). Since it’s unlikely that a human would stay awake for that long, researchers have noted that it’s more likely that total sleeplessness would be indirectly fatal before the 11-day mark, through something like a car crash caused by spontaneously falling asleep at the wheel.

That said, scientists are pretty sure sleep in-and-of-itself is vital, in part because it’s widely believed to be something all animals do (paywall). “Sleep…is playing a vital function in our and other species’ survival–even if we don’t know for sure what that function is at this stage,” says Nadine Gravett, a neuroscientist studying sleep at University of the Witwatersrand in South Africa.

Recent research from the California Institute of Technology found (paywall) that the Cassiopea genus of jellyfish appear to sleep. That’s significant because these sea creatures have no brains, as we traditionally understand them. Whereas most animals keep their neurons all bundled up together in the skull, these “upside-down” jellyfish have neurons throughout their bodies. Given that they’ve been on the planet 600 million years (compared to our puny 2.8 million years), it looks like sleep, or something similar to it, is so essential to living that even the most basic lifeforms have to do it.

In this context, the specific functions of sleep seem to matter less. Who cares if we know every single thing that happens to our body during this strange time we spend completely off the sensory grid? What matters is that it’s something evolution—which only cares about survival—has clung to with the same tight grip as as it has eating and reproducing.

Perhaps we should focus instead on why we aren’t sleeping enough. Matthew Walker, director of Center for Human Sleep Science at the University of California-Berkeley, thinks the problem is largely cultural. In most parts of the world in 2017, humans have lights to trick their brains into believing “waking” hours are longer than nature intended. Contemporary humans also have access to stimulants like caffeine to temporarily alleviate sleepiness.

Then there’s the pressure to keep up with demanding jobs and full social lives (and also, Netflix). “We have stigmatized sleep with the label of laziness,” Walker told the Guardian in Sept 2017. “We want to seem busy, and one way we express that is by proclaiming how little sleep we’re getting. It’s a badge of honor.”

Walker thinks that the solution is to change the way we contemplate sleep. Instead of thinking of it as something that we should do to be our sharpest, healthiest selves, we need to think of it like an obligation. “People use alarms to wake up,” Walker said. “So why don’t we have a bedtime alarm to tell us we’ve got half an hour, that we should start cycling down?”

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