How We Learn (28 page)

Sleep is the stuff of legends and fairy tales precisely because it’s so unknown, a blank screen onto which we can project our anxieties and hopes. If the darkroom is locked, we can only guess at what images are being developed in there. All of which raises the question: What is the sleeping brain doing, exactly?

For that matter, why do we sleep at all?

The truth is, no one knows. Or, to be more precise, there’s no single, agreed-upon scientific explanation for it. We spend fully a third of our existence unconscious, so any theory about sleep’s central purpose has to be a big one. Doesn’t the body need regular downtime to heal? To relieve stress? To manage moods, make muscle, restore mental clarity? Yes to all of the above. We know that sleep
deprivation makes us more reckless, more emotionally fragile, less able to concentrate and possibly more vulnerable to infection. None of those amounts to an encompassing theory, though, because none explains the vast variations in sleep times and schedules. Just think of how dramatically sleep habits differ from person to person. Some people thrive on as little as three hours a night, while others feel helpless without eight; some function best awake all night and out most of the day; others need their daily nap. A truly comprehensive theory of sleep, then, would have to explain such differences. It would also need to account for the sleep-wake cycles in animals, which is breathtaking in its diversity. Female killer whales can be mobile and alert for upward of three weeks when looking after a newborn calf—nearly a month without sleep. Migrating birds fly for weeks without
stopping to rest.

Two new theories have emerged that make sense of this chaos.

One is that sleep is essentially a time-management adaptation. Our body’s internal clock evolved to keep us out of circulation when there’s not much of a living to be made—at 3
A.M.
, for instance—and awake when there is. Consider the brown bat, perhaps the longest-sleeping mammal of them all. It sleeps twenty hours a day and spends the other four, at dusk, hunting mosquitoes and moths. Why only four hours at dusk? Because that’s when food is plentiful. But also because, as Jerome Siegel, a neuroscientist at UCLA, says, “increased waking time would seem to be highly maladaptive for this animal, since it would expend energy and be exposed to predatory birds with better vision and
better flight abilities.” Siegel argues that our obsession with sleep quality and duration is, in a sense, backward. “We spend a third of our life sleeping, which seems so maladaptive—‘the biggest mistake nature has made,’ scientists often call it,” he told me. “Another way of looking at it is that unnecessary wakefulness is a bigger mistake.”

When there’s hay to be made, we make it, whether the sun is shining or not. And when there’s none—or too little, given the risks of
being out and about—we bed down. In short: Sleeping and waking adjust themselves to the demands and risks of our life, not according to what the health manuals say.

The other theory is that sleep’s primary purpose is memory consolidation. Learning. In recent years, brain scientists have published an array of findings suggesting that sleep plays a critical role in flagging and storing important memories,
intellectual and physical. Also (yes) in making subtle connections—a new way to solve a tricky math problem, for example, or to play a particularly difficult sequence of notes on the viola—that were invisible during waking. Think about what we described back in
chapter 1
, all those streaming sensations, the sheer, insane volume of neural connections the brain has to make in the course of any given day. At some point, we have to decide which of these connections are worth holding on to, and which can be ignored. That’s an easy choice sometimes, and we make it immediately: a new colleague’s name; the pickup time at day care; which house on the street has the angry Dobermans. Other choices are not obvious at all. Some of the most critical perceptions we register in a day contain subtle clues—shrugs, sideways glances, suggestions, red herrings. A world of impressions swirls in our heads when we turn the lights out and, according to this theory, that’s when the brain begins to sort out the meaningful from the trivial.

In the contentious field of sleep research, these two theories are typically set in opposition, one trumping the other as the primary function of our unconscious lives. In reality, they are hardly mutually exclusive. Only by putting them together, in fact, can we begin to understand how sleep aids learning—and to use that understanding to our advantage.

• • •

The boy’s brain was going haywire but he was fast asleep, out cold. His father called his name:
Armond? Armond?
No response. Was he pretending? No, it sure didn’t look that way.

It was December 1951, and Eugene Aserinsky, a young graduate student at the University of Chicago, had brought his eight-year-old son, Armond, to his basement lab to perform an
experiment on sleep. Aserinsky was studying for a degree in physiology and trying to build his credentials as an experimental scientist; he had little interest in sleep research as a career. He was only here pulling night duty, on orders from his academic advisor, Nathaniel Kleitman, who happened to be the father of modern sleep science. Aserinsky had been tinkering with a machine called an Offner Dynograph to track the sleeping brain. A forerunner to the EEG, the Dynograph registers electrical signals from the brain, through electrodes taped to the skull. Aserinsky was using Armond as his test subject. He’d taped a couple of electrodes to the boy’s head and eyelids (to track their motion) and then tuned the machine from the next room, asking his son to look this way and that, calibrating the dials. Gradually, Armond nodded off and Aserinsky, sipping his coffee, watched as the Dynograph settled, its ink pens tracing smaller, smoother waves, as expected. But after a few hours the waves began to spike—all of them, those coming from Armond’s eyelids as well as his brain—as if the boy was awake and alert. Aserinsky got up from his chair and slipped into the room where his son lay, to make sure his son was asleep and safe.

Armond?
…
Armond?
No answer.

Aserinsky returned to the next room, and watched the Dynograph. Scientists at the time considered sleep a period when the brain essentially shut down, becoming a playground for the unconscious, a canvas for dreams. The Dynograph said differently. Aserinsky paced the lab—“flabbergasted,” he would say later, by the frenzied wave activity—and watched as Armond’s brain waves settled down again, the pens ceasing their chatter. It was late, there was no one else around. Was he seeing things? If so, then reporting the finding would be potentially embarrassing, written off as the misplaced exuberance of an inexperienced researcher. If not, his son’s
sleeping brain could be telling him something that no one suspected about unconsciousness.

He brought Armond back into the lab for another session weeks later, to see if his original observation was a fluke. It wasn’t. At various periods during the night, Armond’s brain leapt to life as if he were wide awake. Aserinsky was now confident that this pattern was no mirage. “The question was, what was triggering these eye movements?” he said years later. “What do they mean?”

He didn’t have enough expertise in the field or its experimental techniques to know. He’d have to go to the top—to Kleitman—and ask whether such odd brain activity had been reported in sleep experiments before, and whether it was worth the time to follow up. Kleitman didn’t hesitate. “Study more people,” he told Aserinsky. “You might be on to something.”

By late 1952, Aserinsky had upgraded his equipment and embarked on a study of two dozen adults. Their brain patterns looked just like Armond’s: periods of slow undulations, punctuated by bursts of intense activity. The flare-ups had no precedent in the sleep research literature, so he wasn’t even sure what to call them. He consulted Kleitman again, and the two of them reviewed the data. If they were going to report such an unusual finding and claim it was universal, they’d better be sure of their measurements.

Their report finally appeared in September of 1953 in
the journal
Science
. The paper was all of two pages, but Aserinsky and Kleitman did not undersell the implications of their work. “The fact that these eye movements, this EEG pattern, and autonomic nervous system activity are significantly related and do not occur randomly suggests that these physiological phenomena, and probably dreaming, are very likely all manifestations of a particular level of cortical activity which is encountered normally during sleep,” they concluded. “An eye movement period first appears about three hours after going to sleep, recurs two hours later, and then emerges at somewhat closer intervals a third or fourth time shortly prior to awakening.” They
eventually settled on a more scientific-sounding name for the phenomenon: rapid eye movement, or REM, sleep.

“This was really the beginning of modern sleep research, though you wouldn’t have known it at the time,” William Dement, then a medical student in Kleitman’s lab and now a professor of psychiatry and sleep medicine at Stanford University, told me. “It took years for people to realize what we had.”

One reason for the delay was lingering infatuation with an old theory. In the 1950s many brain scientists, particularly in the United States, were still smitten with Freud’s idea that dreams are wish fulfillment, played out in fantasy and symbolic imagery that’s not accessible during waking. Money poured into sleep research but it was used to investigate the
content
of dreams during REM, not the mechanics or purpose of REM per se—and to little avail. People roused from REM described a tangle of anxieties, fantasies, and nonsense scenes that said nothing consistent about human nature. “It was exciting work to do, but in the end we weren’t able to say anything conclusive,” Dement told me. Still, those dream studies and others confirmed beyond any doubt that REM was universal and occurred periodically through the night, alternating with other states of unconsciousness. In fact, people typically experience four or five bursts of REM during the night—of twenty to thirty minutes in duration—as the brain swims up to the brink of consciousness before diving back down again. By 1960, sleep scientists began to speak of sleep as having at least two dimensions: REM and non-REM, or NREM.

Later, using EEG recordings as well as more specific electrical recordings from the eyes and eyelids, researchers found that NREM has its own distinct stages as well. The definition of these stages is arbitrary, depending mostly on the shape and frequency of the waves. The light sleep that descends shortly after we doze off was called Stage 1; this is when the brain’s jagged waves of conscious awareness begin to soften. In Stage 2, the waves become more regular,
resembling a sine wave, or a clean set of rollers moving toward shore on a windless day. In Stages 3 and 4, the waves gradually stretch out, until they undulate gently like a swell over open ocean, a slow-wave pattern that signals the arrival of deep sleep. The brain cycles though its five sleep stages in order: from Stage 1 down to Stage 2, deeper to Stage 3, and bottoming out at Stage 4, before floating back up, through Stages 3 and 2, and then into REM. The cycle then repeats throughout the night, dropping down again to Stage 4 and back up, to REM. These four stages and REM describe what scientists call sleep architecture, which maps easily onto a graph:

The discovery and description of this previously hidden architecture did more than banish the notion, once and for all, that our brains simply “power down” at night, becoming vessels for dreams. It also begged a question: If the brain is so active while we sleep, what’s it up to, exactly? Nature doesn’t waste resources on this scale. With its bursts of REM and intricate, alternating layers of wave patterns, the brain must be up to
something
during sleep. But what?

“To do science, you have to have an idea, and for years no one had one,” J. Allan Hobson, a psychiatry professor at Harvard, told me. “They saw sleep as nothing but an annihilation of consciousness. Now we know different.”

• • •

One reason that palace intrigue makes for such page-turning fiction or addictive TV is what psychologists call “embedded hierarchy.” The king is the king, the queen the queen, and there are layers of princes, heirs, relatives, ladies-in-waiting, meddling patriarchs, ambitious newcomers, and consigliere types, all scheming to climb to the top. Which alliances are most important? What’s the power hierarchy? Who has leverage over whom? You have no idea until you see the individuals interact. And if you don’t see them square off one-on-one, you play out different scenarios to see if you can judge the players’ relative power. Could Grishilda have Thorian shackled and tossed in the moat if the two clashed? She is a favorite of the king’s, after all. Yet Thorian might have some connections up his sleeve … wait, who’s his mother again?

Learning scientists like embedded hierarchy problems because they model the sort of reasoning we have to do all the time, to understand work politics as well as math problems. We have to remember individual relationships, which is straight retention. We have to use those to induce logical extensions: if A > B and B > C, then A must be > C. Finally, we need to incorporate those logical steps into a larger framework, to
deduce
the relationships between people or symbols that are distantly related. When successful, we build a bird’s-eye view, a system to judge the relationship between any two figures in the defined universe, literary or symbolic, that’s invisible to the untrained mind.