| The Memory Issue | Autumn 2008 |
Dream Weaver Perhaps, he mused, those recurring images and sensations were his brain’s attempt to help his body remember how to traverse the mountain. When he returned to Boston, Stickgold shared his suspicion with his students: that sleep-onset, or hypnagogic, dreams—the thoughts and images that run through our minds just as we fall asleep—play an important role in memory and learning. Studying the idea, however, posed a challenge. Stickgold knew he couldn’t just take a group of subjects mountain climbing. His students suggested a simpler—and safer—solution: Tetris, an addictive computer game that involves the repeated positioning of falling shapes into flush lines. Those who had played the game often remembered dreaming about it later. In the study that ensued, 27 participants played Tetris on and off for three days. Stickgold’s team then monitored their hypnagogic dreams. All of those who recalled their dreams reported seeing the same falling Tetris shapes. They also improved their Tetris–playing skills over the course of the study. Yet the findings, published nearly a decade ago, remain significant not only for showing what dreams the subjects had, but who had them. Five amnesiacs had been included in the study, mostly as a nod to a student who wanted to research amnesia but couldn’t obtain funding. Surprisingly, three of the five also reported dreaming of falling shapes. And while the amnesiacs didn’t improve as much as the other subjects, at least one knew which computer keys to press—even though she couldn’t recall ever having played the game. “This,” says Stickgold, “suggests that the hippocampus, which is damaged in amnesiacs, isn’t the part of the brain being used to construct hypnagogic dreams.” It may also explain why hypnagogic dreams aren’t just episodic memories, which are controlled by the hippocampus. If they were, subjects would remember more details of the original experience, such as the room in which they played Tetris. The fact that they don’t dream about these details suggests what Stickgold calls “a brilliant evolutionary gimmick”: Our brains pinpoint the most important information—repetitive activities such as positioning the falling shapes or climbing the mountain—and discard the rest. “It may be that sleep has evolved to extract meaning and improve learning,” explains Stickgold. “That’s why dreams are always meaningful, even when they seem silly to us.” Dreams that occur during deeper, rapid eye movement (REM) sleep also have something to teach us, although the lessons may be less obvious. Imagine you’re driving through an intersection and another car runs a red light. There’s no collision, but it’s a close call. That night, you dream you’re playing bumper cars with your son and you’re feeling unhappy. The message from your brain, says Stickgold, isn’t that you hate amusement parks, but that you should be more vigilant behind the wheel. Because the hippocampus is not involved in REM sleep, the brain is unable to access episodic memories during this phase. The resulting dreams, while seemingly bizarre, still serve as warnings and lessons. “The real purpose of the relationship between memory and sleep,” he says, “may be to inform us not about our past but about our future.” Stickgold’s team continues to investigate that relationship. One study involves playing a recording of related words—such as bed, night, and snooze—for subjects, then testing them later on their recall. The word subjects tend to remember most—sleep—isn’t even on the list. The implication, he says, is that our brains fill in gaps to retain the gist of something important, particularly after a night’s sleep. Other research by Stickgold’s team suggests that the brain requires three stages for optimal memory recall—and that adequate sleep is integral to this process. During the first stage, the brain forms a memory, a phase that depends on about six waking hours to occur. If we nod off too soon, we lose that memory. For the second stage, we need at least several hours of shuteye to edit memories and sharpen learning. In the third stage, edited memories become stabilized and enhanced, allowing the brain to perfect previously learned skills. Perhaps one of the more intriguing areas of research on Stickgold’s agenda is the link between sleep disturbances and mental health. “Every major psychiatric disorder has sleep disturbance as one of its characteristics—and it’s assumed that mental illness causes these disturbances,” he says. “But I believe the relationship is at least bidirectional.” He points to a study he’s published that found schizophrenics who learn a simple task appear to show no overnight improvement in that skill, a possible sign that they may not process memories properly during sleep. Stickgold hopes to someday discover what function sleep and its effects on memory and learning may have in conditions such as depression, bipolar disorder, and attention deficit hyperactivity disorder. So how much sleep do we need for optimal memory? “Simply put, we just don’t know,” Stickgold says. Some studies have found a 90-minute afternoon nap to be more effective than six hours of nighttime sleep at improving learning; other research suggests that consistency over time may be more important than the amount of sleep we get on any given night. One thing is clear: Sleep is necessary for optimal brainpower. Without a full night’s sleep, memory, attention, focus, judgment, reaction time, and learning all suffer. Studies by Charles Czeisler, the Baldino Professor of Sleep Medicine at HMS, have graphically illustrated those consequences in medical residents: fatigue-related medical errors, needle sticks, and motor vehicle crashes. Shuteye has other benefits, too. There’s evidence linking sleep deprivation to decreased immunity and weight gain. The bottom line, says Stickgold, is that some amount of sleep is essential for good health. “The most depressing thing,” he laughs, “is that I’m spending my entire career proving that my mother was right—about sleep, anyway.” Jessica Cerretani is assistant editor of the Harvard Medical Alumni Bulletin Photo: Ann Cutting/Photonica/Getty Images |
But Robert Stickgold, a sleep researcher and an associate professor of psychiatry at HMS, wasn’t on Camel’s Hump; he had reached the summit of the Vermont peak earlier that day. He was asleep in bed—but the memory of his trek wouldn’t fade. Even when he awoke during the night and went back to sleep, “I still found myself dreaming of the climb,” he says. “I had the profound sense that this was somehow important.”
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