The Third of Life We Forgot to Understand

It is 2:13 a.m. on a Tuesday in the autumn of 2024, and somewhere in a small apartment, a thumb slides across a glowing rectangle of glass. The phone has been picked up eleven times in the last hour. Each time, the screen offers the same small consolation: another article, another message, another reel of a stranger laughing. The body beneath the thumb is exhausted. Its eyes sting. Its shoulders have curled toward the ears. The brain behind the eyes has been awake for nineteen hours, and according to almost every biological measure available to modern science, it is performing badly. Reaction time is impaired. Emotional regulation is fraying. The hippocampus, a structure crucial for memory consolidation, has been quietly shutting down processes that can only run during a state the body has not been allowed to enter for nearly a full day. And yet the thumb scrolls, because somewhere in the back of the mind is a quiet, almost unconscious assumption that sleep is something to be rationed, postponed, or stolen back later in fragments. It is, in the cool logic of the late hour, just downtime.

That assumption is wrong. It is wrong in a way that touches every human being who has ever lived and almost every animal that has ever had a nervous system. It is wrong in a way that science has been slow, very slow, to correct. And the correction, when it finally came in the twentieth century, did not arrive gently. It arrived through electrodes pasted to scalps, through students willing to be locked in caves for a month, through the patient observation of closed eyelids, and through one young researcher who first noticed that the eyes of sleeping infants were not still at all. The story of how humanity went from dismissing a third of life as a kind of absence to recognizing it as one of the most active, mysterious, and necessary states of the body is the story this chapter is going to tell. It is, in a sense, the story of how we learned to look.

To understand why this history matters, you have to begin with a simple, almost embarrassing question. Why has it taken us so long? Sleep is not a marginal phenomenon. Every person you will ever meet has done it. The same was true of every ancestor stretching back tens of thousands of years, and of every cousin species in the mammalian lineage, and of nearly every animal with a nervous system at all. The cost of getting sleep wrong is enormous; the cost of getting it spectacularly wrong is death. And yet for most of recorded human history, our species has treated sleep with a kind of intellectual shrug. It was there. It happened. It was, like digestion or breathing, something the body did on its own and that thinking minds need not bother themselves with. The fact that this shrug persisted for thousands of years tells us something profound about how human beings relate to the parts of their own experience that cannot easily be observed from the outside.

What changed, in the last century, was observation. Specifically, it was the invention of a technology that could eavesdrop on the sleeping brain without waking it, and a small group of researchers stubborn enough to ask what was happening in there. The names of these researchers are not household ones, but their discoveries are. Nathaniel Kleitman spent thirty-two days in Mammoth Cave to test the limits of human rhythm. Eugene Aserinsky, working in Kleitman's lab, was the first to notice the rapid eye movements that gave a name to a stage of sleep nobody had suspected. William Dement turned those observations into the first taxonomy of sleep. Allan Rechtschaffen, decades later, would perform experiments so cruel in their logic that they would force the scientific community to take seriously the idea that sleep is not optional. Together, these figures—along with many others—transformed sleep from a vague interval of rest into a measurable, structured, biologically essential process.

This chapter traces that transformation. It begins with the universality of sleep across the animal kingdom, moves through the long historical period during which sleep was treated as passive downtime, follows the invention of the electroencephalogram and the resulting revolution, and ends with the cultural inheritance we still carry today—an inheritance in which the 2 a.m. phone-scroller feels faintly guilty but cannot quite say why. The goal is not merely to give you a timeline. The goal is to give you a vocabulary. Once you have a vocabulary for what sleep actually is, the question of how to treat it begins to answer itself.

The Sleep We All Share

There is a small but telling fact about sleep that almost never gets the attention it deserves. Of all the major behaviors studied by biologists—eating, mating, fighting, fleeing, communicating—sleep is the only one that appears in essentially every animal with a nervous system. Flies sleep. Honeybees sleep. Octopuses sleep, and they appear to dream, their skin rippling through colors and patterns that resemble the chromatophore displays of waking life. Zebrafish sleep. Elephants sleep, though for only two to four hours a day, often while standing. Giraffes sleep in tiny bursts of a few minutes at a time, totalling less than two hours in twenty-four. Brown bats sleep for twenty hours. Dolphins and some species of whale sleep with one half of the brain at a time, allowing the other half to keep surfacing for air.

If you map sleep across the tree of life, two things become immediately apparent. First, the behavior is ancient. Its evolutionary roots reach back at least to the common ancestor of vertebrates and many invertebrates, which means sleep in some form has been around for hundreds of millions of years. Whatever sleep is, it is not a recent invention. Second, it is conserved. Species do not slowly evolve away from it. They modify it, sometimes dramatically—dolphins unihemispherically, giraffes polyphasically, brown bats to a near-vegetative extreme—but they do not abandon it. There are essentially no examples of a complex nervous system that has eliminated sleep entirely.

This is significant, because most behaviors that are conserved across such a wide range of species are behaviors whose absence quickly leads to death. An animal that stops eating dies in weeks. An animal that stops drinking dies in days. An animal that stops sleeping dies in weeks as well, as Rechtschaffen's experiments would dramatically demonstrate more than fifty years after sleep science had begun in earnest. The conservation of sleep, in other words, is the conservation of something the nervous system cannot do without.

Yet the universality of sleep makes its historical neglect all the more puzzling. The phenomenon that unites virtually every animal with a nervous system is the very phenomenon that human beings, until the twentieth century, were most content to ignore. We had explanations for it, of course. They just happened to be wrong. Aristotle thought sleep was caused by vapors rising from the stomach after a meal, condensing in the brain, and weighing the body down toward unconsciousness. The Romans had similar ideas, mixed with the belief that sleep was a kind of small death, a nightly visit from the realm of the dead. Medieval European physicians treated sleep as a humoral process, balancing the four cardinal fluids of the body. None of these explanations were unreasonable for their time. They were all, however, explanations of what sleep *did* to the body, not what sleep *was* for the body. They were, in modern terms, descriptions without functions. They told you about the mechanism (or what passed for mechanism), but they did not tell you why the body bothered.

The reason for this neglect is itself a fascinating story, and it has to do with the difficulty of observing a state in which the observer cannot participate. To study sleep scientifically, you have to be able to look inside a sleeping brain without waking it up. That technology did not exist