Why Do We Sleep?
This is the first installment of a five-part series.
Why the obsession? And what, exactly, is sleep for? A little more than a half-century ago, most scientists believed that sleep was an inactive state, a kind of parenthesis in living. Then in 1951, Eugene Aserinsky, a clever graduate student at the University of Chicago, hooked his son Armond to a retooled "brain wave machine" and monitored the boy's sleep deep into the night. Aserinsky observed sharp spikes of activity on his readout, suggesting that Armond's eyes were darting back and forth. This turned out to reflect the distinctive state within sleep dubbed rapid eye movement, or REM— a "new continent in the brain," as a colleague later put it. Aserinsky and his adviser, Nathaniel Kleitman, hypothesized that rapid eye movement was related to dreaming. Soon Kleitman and others were mapping the basic cartography of sleep, which alternates between non-REM and REM periods throughout the night, in roughly 90-minute cycles.
Sleep is not an optional enterprise. All mammals do it. So do birds, reptiles, and even fruit flies. Rats deprived of sleep apparently die faster than those deprived of food. Sleep deprivation is a ruthlessly effective means of torture, as the new movie The Lives of Others shows in a stomach-turning scene. Yet the bedrock question—what purpose does sleep serve for us and the rest of the animal kingdom—remains oddly unsettled.
After sleeping on it, I went in search of some theories. I started with Robert Stickgold, a cognitive neuroscientist at Harvard, whose office is located in one of the quieter precincts of Beth Israel Deaconess Hospital, in what used to be a patient room. "There's an old joke that the function of sleep is to cure sleepiness," he says, smiling, white hair mussed and thick eyebrows animated. "We think we can deprive ourselves of sleep and drink triple espressos or take Modafinil. But that's like saying, 'I've figured out how to cure hunger in Africa: I'm going to send over amphetamines so people won't feel hungry anymore.' It doesn't address the underlying need."
Except that with sleep, as opposed to food, no one knows exactly what the underlying need is. Stickgold is one of the foremost sleep researchers in the country and has long argued that sleep's crucial function is to boost memory and learning. His theory is that during sleep, the brain evaluates recently learned information and decides what to do with it. In the process, memory consolidation takes place—memories or skills that were acquired during waking are stabilized or enhanced, or perhaps moved to new locations. The brain may also extract patterns and rules from large amounts of information.
Evidence that sleep plays a role in memory consolidation and mental processing comes largely from behavioral research in which scientists teach people a task and then try to pinpoint a boost in performance that they can attribute to sleep. Frequently, they try to correlate the gain with a particular portion of sleep, as well—REM versus non-REM. In an experiment published in 2000, for instance, Stickgold and his colleagues asked subjects to identify, as rapidly as possible, the orientation of diagonal bars against a background of horizontal ones. Some people returned for retesting later the same day and showed little improvement. Others returned after a night of sleep and did significantly better. The sleepers' improved scores were proportional to the amount of slow-wave sleep (a stage of non-REM) they had gotten early in the night and the amount of REM sleep they had gotten late in the night. The researchers also found that when subjects were deprived of sleep the night after learning the task, their performance did not significantly improve later, even after two subsequent nights of recovery sleep. And so they concluded that sleep "within 30 hours of training is absolutely required for improved performance."
Sleep may facilitate more complex forms of insight as well. For a study reported in 2004, a German group asked subjects to work on a series of interrelated math problems, which contained a "hidden rule" that allowed them to be solved more quickly. People who returned to the puzzles after a night of sleep found the shortcut roughly twice as often as those who had spent an equal amount of time awake. What enabled the flash of insight, the researchers suggested, may have been the "restructuring of representations in memory" during sleep.
New work from Stickgold's lab also suggests that sleep helps people to extract themes or meaning from information they were presented with during the day. When shown lists of related words, for instance, subjects were better able to recall the unifying principle of the list—what the words had in common—after sleeping.
Stickgold is sure that sleep is crucial to "how our memories are initially stabilized and ultimately shaped," including how new memories are integrated with older ones. And this function may help to explain sleep's evolution. During sleep, animals cannot hunt for food or produce more offspring and may be more vulnerable to predators. So to have endured all these millennia, snoozing is likely to offer some adaptive advantage that outweighs its risks. But what? Stickgold suggests that the different stages of sleep—with their distinctive patterns of brain activity—were selected for because they help the brain to perform different kinds of memory tasks.
Yet questions remain. In some of the older work on sleep and memory, methodological issues make it hard to tell if gains in performance are due specifically to sleep (as opposed to, say, the passage of time). More recently, there has been a huge push to tackle which stages of sleep enhance memory—and which sorts of memory get a critical boost from sleep. When you sort through them, the distinctions that crop up can sometimes seem a little baroque. Why should learning to discriminate among visual patterns depend on slow-wave sleep early in the night and REM sleep late, as Stickgold has shown? Are there more thoroughgoing effects throughout the night that we have not yet picked up on? Also, apparent contradictions emerge with regard to particular sleep stages. For instance, as this review points out, one study found that depriving people of all sleep in the second half of the night (when REM predominates) impaired their improvement on a given task. But depriving them of REM sleep for the entire night did not have the same effect.
Maybe future research will resolve all of this. Or perhaps we'll learn that the mapping of specific memory tasks onto portions of sleep tells only a partial story. The effects of sleep on memory could turn out to be byproducts of something more basic—as the slumber of peculiar creatures like the platypus hints. More on that tomorrow.
Amanda Schaffer is a science and medical columnist for Slate.