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.
As we fall into sleep, our brain stays active and fires into its editing process—deciding which memories to keep and which ones to toss.
The initial transformation happens quickly. The human body does not like to stall between states, lingering in doorways. We prefer to be in one realm or another, awake or asleep. So we turn off the lights and lie in bed and shut our eyes. If our circadian rhythm is pegged to the flow of daylight and dark, and if the pineal gland at the base of our brain is pumping melatonin, signaling it’s nighttime, and if an array of other systems align, our neurons swiftly fall into step.
Neurons, some 86 billion of them, are the cells that form the World Wide Web of the brain, communicating with each other via electrical and chemical signals. When we’re fully awake, neurons form a jostling crowd, a cellular lightning storm. When they fire evenly and rhythmically, expressed on an electroencephalogram, or EEG, by neat rippled lines, it indicates that the brain has turned inward, away from the chaos of waking life. At the same time, our sensory receptors are muffled, and soon we’re asleep.
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.
The strength of one’s nightly spindles, some experts have suggested, might even be a predictor of general intelligence. Sleep literally makes connections you might never have consciously formed, an idea we’ve all intuitively realized. No one says, “I’m going to eat on a problem.” We always sleep on it.
The waking brain is optimized for collecting external stimuli, the sleeping brain for consolidating the information that’s been collected. At night, that is, we switch from recording to editing, a change that can be measured on the molecular scale. We’re not just rotely filing our thoughts—the sleeping brain actively curates which memories to keep and which to toss.
It doesn’t necessarily choose wisely. Sleep reinforces our memory so powerfully—not just in stage 2, where we spend about half our sleeping time, but throughout the looping voyage of the night—that it might be best, for example, if exhausted soldiers returning from harrowing missions did not go directly to bed. To forestall post-traumatic stress disorder, the soldiers should remain awake for six to eight hours, according to neuroscientist Gina Poe at the University of California, Los Angeles. Research by her and others suggests that sleeping soon after a major event, before some of the ordeal is mentally resolved, is more likely to turn the experience into long-term memories.
Stage 2 can last up to 50 minutes during the night’s first 90-minute sleep cycle. (It typically occupies a smaller portion of subsequent cycles.) Spindles can arrive every few seconds for a while, but when these eruptions taper off, our heart rate slows. Our core temperature drops. Any remaining awareness of the external environment disappears. We commence the long dive into stages 3 and 4, the deep parts of sleep.
We enter a deep, coma-like sleep that is as essential to our brain as food is to our body. It’s a time for physiological housekeeping—not for dreaming.
Every animal, without exception, exhibits at least a primitive form of sleep. Three-toed sloths snooze about 10 hours a day, a disappointing display of languor, but some fruit bats manage 15 hours, and little brown bats have been reported to laze for 20. Giraffes sleep less than five. Horses typically sleep part of the night standing up and part lying down. Dolphins sleep one hemisphere at a time—half the brain sleeps while the other half is awake, allowing them to swim continuously. Great frigatebirds can nap while gliding, and other birds may do the same. Nurse sharks rest in a pile on the ocean floor. Cockroaches lower their antennae while napping, and they’re also sensitive to caffeine.
Sleep, defined as a behavior marked by diminished responsiveness and reduced mobility that is easily disrupted (unlike hibernation or coma), exists in creatures without brains at all. Jellyfish sleep, the pulsing action of their bodies noticeably slowing, and one-celled organisms such as plankton and yeast display clear cycles of activity and rest. This implies that sleep is ancient and that its original and universal function is not about organizing memories or promoting learning but more about the preservation of life itself. It’s evidently natural law that a creature, no matter the size, cannot go full throttle 24 hours a day.
“Being awake is demanding,” says Thomas Scammell, a neurology professor at Harvard Medical School. “You’ve got to go out there and outcompete every other organism to survive, and the consequences are that you need a period of rest to help cells recuperate.”
For humans this happens chiefly during deep sleep, stages 3 and 4, which differ in the percentage of brain activity that’s composed of big, rolling delta waves, as measured on an EEG. In stage 3, delta waves are present less than half the time; in stage 4, more than half. (Some scientists consider the two to be a single deep-sleep stage.) It’s in deep sleep that our cells produce most growth hormone, which is needed throughout life to service bones and muscles.
There is further evidence that sleep is essential for maintaining a healthy immune system, body temperature, and blood pressure. Without enough of it, we can’t regulate our moods well or recover swiftly from injuries. Sleep may be more essential to us than food; animals will die of sleep deprivation before starvation, says Steven Lockley of Brigham and Women’s Hospital in Boston.
Good sleep likely also reduces one’s risk of developing dementia. A study done in mice by Maiken Nedergaard at the University of Rochester, in New York, suggests that while we’re awake, our neurons are packed tightly together, but when we’re asleep, some brain cells deflate by 60 percent, widening the spaces between them. These intercellular spaces are dumping grounds for the cells’ metabolic waste—notably a substance called beta-amyloid, which disrupts communication between neurons and is closely linked to Alzheimer’s. Only during sleep can spinal fluid slosh like detergent through these broader hallways of our brain, washing beta-amyloid away.
This article was originally posted on National Geographic
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