All animals seem to sleep. Even quite simple creatures like nematodes and fruit flies have periods of quiescence. The amount of sleep needed varies markedly among animals. Elephants and horses get by on just two or three hours a night. Why they need so little is unknown. Most other mammals require a great deal more. The animal that used to be thought the mammalian sleep champion, the three-toed sloth, is still often said to sleep for up to twenty hours a day, but that number came from studying captive sloths, who have no predators and not a lot to do. Wild sloths slumber for more like ten hours a day—not a huge amount more than we do. Extraordinarily, some birds and marine mammals are able to switch off one half of their brain at a time, so that one half remains alert while the other is snoozing.
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Our modern understanding of sleep may be said to date from a December night in 1951 when a young sleep researcher at the University of Chicago named Eugene Aserinsky tried out a machine for measuring brain waves that his lab had acquired. Aserinsky’s volunteer subject for the first night’s test was his eight-year-old son, Armond.
Ninety minutes after young Armond had settled down into what was normally a peaceful night’s sleep, Aserinsky was surprised to see the monitor’s unspooling graph paper jerk to life and begin the kinds of jagged tracings associated with an active, wakeful mind. But when Aserinsky went next door, he found Armond still fast asleep. His eyes, however, were moving visibly beneath his lids. Aserinsky had just discovered rapid eye movement sleep, the most interesting and mysterious of the multiple phases of our nightly sleep cycle. Aserinsky didn’t exactly rush the news into print. Almost two years passed before a small report on the discovery appeared in the journal Science.*1
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We now know that a normal night’s sleep consists of a series of cycles, each involving four or five phases (depending on whose categorization methods you favor). First comes the business of relinquishing consciousness, which for most of us takes between five and fifteen minutes to achieve fully. This is followed by a period in which we slumber lightly but restoratively, as in a nap, for about twenty minutes. Sleep is so shallow in these first two stages that you may be asleep but think you are awake. Then comes a deeper sleep, lasting about an hour, from which it is much harder to rouse a sleeper. (Some authorities divide this period into two stages, giving the sleep cycle five distinct periods rather than four.) Finally comes the rapid eye movement (or REM) phase, when we do most of our dreaming.
During the REM part of the cycle, the sleeper becomes mostly paralyzed, but the eyes dart about beneath closed lids as if witnessing some urgent melodrama, and the brain grows as lively as at any time during wakefulness. In fact, some parts of the forebrain are livelier during REM sleep than when we are fully conscious and moving around.
Why the eyes move during REM sleep is uncertain. One obvious idea is that we are “watching” our dreams. Not all of you is paralyzed during the REM phase. Your heart and lungs continue to function, for obvious reasons, and clearly your eyes are free to swivel, but the muscles that control bodily movement are all restrained. The explanation most often proposed is that immobilization stops us from harming ourselves by thrashing about or trying to flee from attack when caught up in a bad dream. A very few people suffer from a condition called REM sleep behavior disorder in which the limbs don’t become paralyzed, and they do indeed sometimes hurt themselves or their partners with their thrashing. For others, paralysis doesn’t immediately abate upon awaking and the victim finds himself awake but unable to move—a deeply unnerving experience, it seems, but one that mercifully tends to last only for a few moments.
REM sleep accounts for up to two hours of every night’s sleep, roughly a quarter of the total. As the night passes, the periods of REM sleep tend to lengthen, so that your most dreamy spells are usually in the final hours before waking.
The cycles of sleep are repeated four or five times a night. Each cycle lasts about ninety minutes, but can vary. REM sleep is seemingly important for development. Newborn babies spend at least 50 percent of their sleep time (which is most of their time anyway) in the REM phase. For fetuses it may be as much as 80 percent. For a long time, it was thought that we did all our dreaming during REM sleep, but a 2017 study at the University of Wisconsin found that 71 percent of people dreamed during non-REM sleep (as compared with 95 percent during REM sleep). Most men have erections during REM sleep. Women likewise experience increased blood flow to the genitals. No one knows why, but it seems not to be overtly associated with erotic impulses. Typically, a man will be erect for two hours or so a night.
We are more restless at night than most of us realize. The average person turns over or significantly changes position between thirty and forty times in the course of a night. We also wake up far more than you might think. Arousals and brief awakenings in the night can add up to thirty minutes without being noticed. On a visit to a sleep clinic for his 1995 book, Night, the writer A. Alvarez thought he had experienced an unbroken night’s sleep but discovered when his chart was reviewed in the morning that he had woken up twenty-three times. He also had five dreaming periods of which he had no recollection.
As well as normal overnight sleep, we also commonly indulge in snatches of wakeful-hours sleep in a state known as hypnagogia, a netherworld between waking and unconsciousness, often without being aware of it. Alarmingly, when a dozen airline pilots on long-haul flights were studied by sleep scientists, almost all were found to have been asleep, or all but asleep, at various times during the flight without realizing it.
The relationship between the sleeping person and the outside world is often a curious one. Most of us have experienced that abrupt feeling of falling while asleep known as a hypnic or myoclonic jerk. No one knows why we have this sensation. One theory is that it goes back to the days when we slept in trees and had to take care not to fall off. The jerk may be a kind of fire drill. That may seem far-fetched, but it is a curious fact, when you think about it, that no matter how profoundly unconscious we get, or how restless, we almost never fall out of bed, even unfamiliar beds in hotels and the like. We may be dead to the world, but some sentry within us keeps track of where the bed’s edge is and won’t let us roll over it (except in unusually drunk or fevered circumstances). Some part of us, it seems, pays heed to the outside world, even for the heaviest sleepers. Studies at Oxford University, related by Paul Martin in his book Counting Sheep, found that EEG readings for test subjects twitched whenever their own names were read aloud as they slept but didn’t react when other, unknown names were recited. Tests have also shown that people are pretty good at waking themselves at a predetermined time without an alarm clock, which means that some part of the sleeping mind must be tracking the real world outside the skull.
Dreaming may simply be a by-product of this nightly cerebral housecleaning. As the brain clears wastes and consolidates memories, neural circuits fire randomly, briefly throwing up fragmentary images, a bit like someone jumping between television channels when looking for something to watch. Confronted with this incoherent flow of memories, anxieties, fantasies, suppressed emotions, and the like, the brain possibly tries to make a sensible narrative out of it all, or possibly, because it is itself resting, doesn’t try at all, and just lets the incoherent pulses flow past. That may explain why we generally don’t remember dreams much despite their intensity—because they are not actually meaningful or important.