Why do we sleep? What science does and doesn’t know

Not getting a good night’s rest stops us from retaining memories and cleansing our brains, and accelerates cognitive decline

A group of children taking a nap in Wuhan, China.
A group of children taking a nap in Wuhan, China.PENG NIAN (Barcroft Media via Getty Images)
Miguel Ángel Criado

In an experiment published in 1995, 200 test subjects stood on treadmills that started up when sensors detected they were beginning to feel sleepy. At the end of the treadmill was a bucket full of water. In the most extreme cases, they were deprived of 99% of their sleep time. After a few days, they began to binge eat, yet they were losing weight. Their metabolic rate skyrocketed by 200% and they developed skin ulcers. The participants’ blood showed abnormal levels of hormones, and within two to three weeks, they were all dead. The research was done on lab rats, of course, but the findings were clear: mammals can’t live without sleep. Science is still having a hard time figuring out exactly why.

The journal Science published a series of articles last week exploring the latest research about sleep. All agreed on the universal need for it. An animal closing its eyes and entering an unconscious state that exposes it to the dangers of predators at night does not seem like a good idea, so from an evolutionary point of view, it must have an essential function. There are a few species that are capable of half-sleeping, such as dolphins. They relax one half of the brain, closing the eye on that side, while the other remains awake. Great frigatebirds are able to sleep in short bursts as they fly for days at a time. But most species need a certain number of hours of sleep, and if they don’t get them one night they will sleep more deeply and for longer the following day.

It’s not possible for ethical reasons to repeat extreme experiments like the rat and treadmill example in humans, nor even with animals these days under new welfare standards. But what we learned from a time when these standards were less strict is that sleep deprivation has a widespread impact on the body, from cognitive abilities to our gait.

Other studies on rats have shown that sleep deprivation caused test subjects to forget how to find the spot where they had obtained food a few hours earlier. Another study in humans demonstrated last year that not sleeping the night before lengthened reaction time at the wheel to a greater degree than those who were over the drunk-driving limit. And a few months ago scientists revealed that the incidence of dementia among 8,000 British civil servants when they retired was higher among those who reported sleeping six or fewer hours a night over the previous decades. Finally, the Massachusetts Institute of Technology (MIT) revealed just last week that students who slept less ran at a slower pace on a treadmill, controlling for other factors.

Nick Franks, professor of Biophysics and Anaesthetics at Imperial College London, contributed to the special issue of Science, and pointed out that sleep’s importance has stood the test of time. “On an evolutionary basis, it seems that sleep has been very well preserved, implying a basic function essential for life. When we are sleep-deprived, all sorts of things related to our health and behavior go wrong, though what the basic mechanism that tracks how tired the brain is, and when sleep should be activated, remains a big mystery,” he said.

In fact, more is known about how bad depriving the brain of sleep is than why it is good for it. This is one of the paradoxes of sleep science: evidence has accumulated about the negative consequences of sleep deprivation, but the true benefits of a good night’s sleep remain underexplained. The simplest and most common answer is that, just like other parts of the body, the brain needs rest after a hard day of receiving all kinds of stimuli. The problem with this analogy is that brain activity during sleep does not stop, but simply changes course.

Gabrielle Girardeau, a researcher at the Institut du Fer-à-Moulin in Paris, conducted the experiment where rats forgot where to go to find food. “In humans, we know that sleep deprivation is detrimental to our memory. In animals, this deprivation also affects memory consolidation,” she said via email. Knowing the negative effects of not sleeping builds the foundations for learning the positive reasons for good sleep hygiene.

Girardeau leads a lab focused on how sleep cements memories in our minds. “Basically, the brain rehearses during sleep what happened during our waking hours,” said the French scientist. “We believe that this process allows the gradual reinforcement of memories over time. In particular, the hippocampus, which is a crucial structure for contextualizing memory (what, where, when), reactivates the brain patterns of our waking hours during sleep, in short coordinated events called ripples.” Girardeau’s team interfered with the rats’ recall by interfering with these ripples. “Ripples help strengthen the marking of memories and also allow the hippocampus to communicate with other parts of the brain, such as the cortex or amygdala, to give emotional value to a memory or to transfer its details to the cortex for long-term storage,” she explained. This crucial process cannot happen if one is awake and being distracted by constant stimuli.

The consolidation of memories is not the only purpose of sleep, although it is perhaps the best explained. Laura D. Lewis specializes in neuroimaging at Boston University’s Biomedical Engineering department, and explores the myriad ways in which sleep affects us. “The neuroscience of sleep has shown that there is no single reason why we sleep: sleep has incredibly broad effects on the brain and affects everything from molecular processes to higher-level cognition,” she said.

A recent area of research for Lewis is showing that one of the functions of sleep is to remove junk from the brain. “Rodent studies have shown that a variety of metabolites [detritus from metabolism] are removed during sleep,” Lewis says. “Many of these metabolites are generated by neurons during wakefulness, when they produce various types of molecules naturally as they consume energy and perform their usual functions. A buildup of one of these, Amyloid beta, “appears related to Alzheimer’s disease,” she added.

This cleansing is a two-step process: during sleep, the waste products of brain activity are evacuated by the cerebrospinal fluid and interstitial fluid, while neurons produce less waste than when awake. Cleaning is done at night, as in office buildings, when the rest of the workers are not there. In this sense, sleep keeps neuron activity healthy.

So if sleep has so many benefits, why do people sleep so little and so poorly? Heather Schofield is an Assistant Professor at the Perelman School of Medicine and The Wharton School at the University of Pennsylvania, and researches the social aspect of sleep. “Most laboratory studies have shown the positive effects of sleep,” she said. But her contribution to the Science special showed that many individuals, whether for work or personal reasons, prefer less sleep. “Some people may decide that it is worth making concessions and sleep less than recommended by experts.”

In Spain, the average adult sleeps for seven hours a night. Dr. Javier Puertas, vice-president of the Spanish Sleep Society, warns that a third sleep less than that amount and that there is also a marked difference between the countryside and the city (urbanites sleep worse). “Sleep has an unproductive image. There is a certain mythology that intelligent, productive, successful people sleep less,” said Puertas. Spain’s late hours don’t help either, and nor does our reliance on screens. “Does anyone remember Un globo, dos globos, tres globos? [a Spanish children’s show from the 1970s that urged kids to go to bed at 8pm]. What child goes to bed at that time these days?”

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