Monday, July 29, 2024

What We Know About Why We Need Sleep

Math and Science News from Quanta Magazine

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Each week Quanta Magazine explains one of the most important ideas driving modern research. This week, biology staff writer Yasemin Saplakoglu explores the molecular underpinnings of sleep and why we need it to survive.

What We Know About Why We Need Sleep

By YASEMIN SAPLAKOGLU

We need sleep. This is a fundamental fact of life. Without it, we turn into grouchy, sluggish zombies. Yet although we do it every day, sleep largely remains a scientific mystery. What happens when we sleep — and what's the point?

We know that a quarter to a third of the human lifespan is spent in slumber. During that time, the brain cycles between two kinds of sleep. In rapid eye movement (REM) sleep, your brain waves are like those you experience while awake. Your brain is active, your eyes are twitching, you dream. During non-REM sleep, however, this activity downshifts: Your body temperature drops, your heart rate slows, your muscles relax and your brain waves slow.

Sleep is common across the animal kingdom. In the 20th century, research in mammals found that the amount of sleep animals need is variable: Armadillos sleep for most of the day, while elephants need only a few hours per night. Those early studies defined sleep in a stringent, mammalian sense, but over the past two decades, researchers have begun to redefine sleep in other creatures, such as fruit flies, worms and zebra fish. They too disconnect from the outside world and sleep in their own ways. That redefinition has opened the field to an array of model organisms to help us better understand what is occurring in our bodies at a molecular level during sleep.

"We know now that genetically sleep is very conserved, and even at the neural level, from the simplest models all the way up to humans," Alex Keene, a neurogeneticist at Texas A&M University, told podcast host Steve Strogatz on The Joy of Why in 2022.

Still, many mysteries surround why we sleep. As scientists dig deeper into the molecular mechanisms of the process, it's become clear that sleep is critical for maintaining a healthy brain and body. It is the time when the brain takes the information we learned while awake and stores it away as long-term memories. Sleep clears out cellular debris accumulated during conscious work. It maintains a healthy metabolism. And not getting enough z's can lead to a suite of issues from diabetes to heart disease. Put simply: Without sleep, we die.

The "why" can be difficult in science. Why do we need sleep to survive? Why do some people need only five hours of sleep while others need eight? Why do we cycle through these two very different phases? Why are so few workers offered siesta time at work? Scientists have only started to unpack some of these questions, but their findings are leading to fascinating new ideas about what happens behind closed eyes.

What's New and Noteworthy

For more than a century, sleep research focused on the brain. But even simple animals such as hydras that don't have brains at all enter a sleeplike state. This means that sleep may have evolved before brains, and its biological role billions of years ago may have been very different from its current one. One hypothesis is that sleep evolved to play a role in metabolism, as Veronique Greenwood reported for Quanta, by allowing certain biochemical reactions to occur that can't happen during waking hours. "Before we had a brain, we had a gut," said Michael Abrams, a fellow at the University of California, Berkeley. Whatever the first sleeping organism was, it probably disappeared more than a billion years ago — and with it, its secrets.

Sleep isn't merely nice to have — we'd be doomed without it. Scientists have known this since the 1890s but haven't been able to figure out why. One explanation is a buildup of toxic molecules in the gut, as Quanta has reported. In the guts of sleep-deprived flies, researchers found an excess of "reactive oxygen species," which damage essential molecules such as DNA and proteins by stealing their electrons. If the flies went without sleep long enough, they were through — but was this toxic pileup causing their deaths? To find out, the researchers gave the flies antioxidants to clear out these chemicals. The critters, though still sleepless, survived. "It was shocking; it was absolutely shocking," said Dragana Rogulja, an assistant professor of neurobiology at Harvard Medical School, on the Joy of Why podcast. It wasn't a one-off: Their team and others have found similar results in rodents.

We know that sleep helps the brain form memories, but scientists are still trying to unpack how. In 2019, Quanta's Jordana Cepelewicz reported that powerful bursts of brain activity, known as sharp wave ripples, are involved in consolidating memories. Recently, I reported that these ripples are also involved in tagging certain experiences while we are awake to be stored as long-term memories while we sleep. An experience registers in the brain in a specific pattern of activity, tapping out a sequence of neurons like "a melody on the piano," Daniel Bendor, a neuroscientist at University College London, told me. When we're awake and resting, the brain replays the pattern to tag it for storage. Then, during sleep, the pattern replays again, hundreds or thousands of times. Some scientists hypothesize that the pattern propagates out toward the cortex and is cemented as a long-term memory.

AROUND THE WEB

Scientific American has investigated why some people report that they haven't slept all night, even though brain scans show that they have — a phenomenon sometimes called subjective insomnia.

Nature Neuroscience published a paper this year suggesting that the function of sleep is to restore an optimal computational state in the brain.