Our floodlit society has made sleep deprivation a lifestyle. But we know more than ever about how we rest—and how it keeps us healthy.

Nearly every night of our lives, we undergo a startling metamorphosis.  Our brain profoundly alters its behavior and purpose, dimming our consciousness. For a while, we become almost entirely paralyzed. We can’t even shiver. Our eyes, however, periodically dart about behind closed lids as if seeing, and the tiny muscles in our middle ear, even in silence, move as though hearing. We are sexually stimulated, men and women both, repeatedly. We sometimes believe we can fly. We approach the frontiers of death. We sleep.

Around 350 B.C., Aristotle wrote an essay, “On Sleep and Sleeplessness,” wondering just what we were doing and why. For the next 2,300 years no one had a good answer. In 1924 German psychiatrist Hans Berger invented the electroencephalograph, which records electrical activity in the brain, and the study of sleep shifted from philosophy to science. It’s only in the past few decades, though, as imaging machines have allowed ever deeper glimpses of the brain’s inner workings, that we’ve approached a convincing answer to Aristotle.
Everything we’ve learned about sleep has emphasized its importance to our mental and physical health. Our sleep-wake pattern is a central feature of human biology—an adaptation to life on a spinning planet, with its endless wheel of day and night. The 2017 Nobel Prize in medicine was awarded to three scientists who, in the 1980s and 1990s, identified the molecular clock inside our cells that aims to keep us in sync with the sun. When this circadian rhythm breaks down, recent research has shown, we are at increased risk for illnesses such as diabetes, heart disease, and dementia.

Yet an imbalance between lifestyle and sun cycle has become epidemic. “It seems as if we are now living in a worldwide test of the negative consequences of sleep deprivation,” says Robert Stickgold, director of the Center for Sleep and Cognition at Harvard Medical School. The average American today sleeps less than seven hours a night, about two hours less than a century ago. This is chiefly due to the proliferation of electric lights, followed by televisions, computers, and smartphones. In our restless, floodlit society, we often think of sleep as an adversary, a state depriving us of productivity and play. Thomas Edison, who gave us light bulbs, said that “sleep is an absurdity, a bad habit.” He believed we’d eventually dispense with it entirely.

A full night’s sleep now feels as rare and old-fashioned as a handwritten letter. We all seem to cut corners, fighting insomnia through sleeping pills, guzzling coffee to slap away yawns, ignoring the intricate journey we’re designed to take each evening. On a good night, we cycle four or five times through several stages of sleep, each with distinct qualities and purpose—a serpentine, surreal descent into an alternative world.

Stages 1-2

Everything we’ve learned about sleep has emphasized its importance to our mental and physical health. Our sleep-wake pattern is a central feature of human biology—an adaptation to life on a spinning planet, with its endless wheel of day and night. The 2017 Nobel Prize in medicine was awarded to three scientists who, in the 1980s and 1990s, identified the molecular clock inside our cells that aims to keep us in sync with the sun. When this circadian rhythm breaks down, recent research has shown, we are at increased risk for illnesses such as diabetes, heart disease, and dementia.

Scientists call this stage 1, the shallow end of sleep. It lasts maybe five minutes. Then, ascending from deep in the brain, comes a series of electric sparks that zap our cerebral cortex, the pleated gray matter covering the outer layer of the brain, home of language and consciousness. These half-second bursts, called spindles, indicate that we’ve entered stage 2.

Our brains aren’t less active when we sleep, as was long thought, just differently active. Spindles, it’s theorized, stimulate the cortex in such a way as to preserve recently acquired information—and perhaps also to link it to established knowledge in long-term memory. In sleep labs, when people have been introduced to certain new tasks, mental or physical, their spindle frequency increases that night. The more spindles they have, it seems, the better they perform the task the next day.

BY MICHAEL FINKEL  – National Geographic

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