Science & Society
The secrets of sleep
It's a mystery, but it clearly makes us smarter and healthier
By Nell Boyce and Susan Brink
USN&WR Issue of May 17, 2004
Health education teacher Pacy Erck remembers what it was like back when Edina High School students had to show up by 7:25 a.m. "The kids were always very tired," she recalls. But these days, Erck rarely has a kid nod off in class. That's because in the fall of 1996, officials at this Minnesota school decided to ring the first bell an hour later, at 8:30 a.m. Sleep researchers had reported that teens' natural slumber patterns favor a later bedtime, and the school wanted to ensure that its high schoolers weren't being shortchanged by an early wake-up call. The change means that students average five more hours of sleep a week, and teachers can see a difference. "You don't have the kids putting their heads down," Erck says. "The class is livelier."
Research confirms real benefits not only at Edina but also at many other high schools that have made similar scheduling switches, says Kyla Wahlstrom, an education policy expert at the University of Minnesota-Twin Cities. Grades have gone up, and dropout rates have declined. The results are impressive enough that other school systems have started to take notice. In Poquoson, Va., the school board has held public hearings over the past few months to consider making the first bell later. "We do believe our children aren't getting as much sleep as they ought to," says Jonathan Lewis, superintendent of schools in Poquoson. "We have children getting up at 5:30, quarter of 6 in the morning."
But what is it about getting more sleep that's actually helping students do better? Is it just that sleepy kids can't concentrate in class because they're dozing off over their books, or does something happen in dreamland that affects the brain's ability to learn and remember?
A growing number of scientists suspect that sleepless students may suffer more than just feeling dragged out during the day. Many intriguing studies in both humans and animals suggest that the sleeping brain does something to solidify memories and process newly learned lessons. The brain work of sleep may even allow people to form insights that they can't achieve while awake, according to research that gives new weight to the old notion of taking a tough problem and "sleeping on it." With most Americans routinely getting far less sleep than they should, some experts are starting to wonder if widespread sleep deprivation is having a real but unrecognized effect on society's brainpower and creativity.
Sleep is clearly important--after all, people and animals slumber away a third of their lives--but no one knows why. The special learning potential of sleep is an idea that has long held sway in the popular culture: Consider those sketchy "learn while you sleep" audiotapes that promise to "tap into the power of your unconscious." But hard scientific evidence has been scant until recently. Most sleep research has focused on health, and sleep has been viewed mainly as a period of rest and rejuvenation for the body.
That's why, for decades, most experts dismissed sleep as a boring, idle time for the brain. What they didn't realize is that while sleeping bodies lie motionless in bed, the brain's neurons continue to buzz and chatter. Only with the 1953 discovery of rapid eye movement (REM) sleep and the development of new machines to monitor brain activity did researchers begin to see what, exactly, was going on at night. Suddenly they could watch as the brain moved through predictable cycles of REM sleep--with its sometimes vivid dream imagery--and deeper "slow wave" sleep.
Before long, some labs noticed that amounts of REM sleep increased as animals learned various tasks but went back to normal after tasks got mastered. What's more, experiments that deprived animals of REM sleep by disrupting them during this sleep stage found that they didn't learn as well as animals that got in plenty of dreaming. But the idea that sleep might aid smarts didn't catch fire. "Sleep wasn't supposed to be for that. It was supposed to be for restoration," recalls Carlyle Smith of Trent University in Ontario, who has studied sleep and learning for over 30 years.
Slowly, though, the idea has attracted more interest, especially since the early 1990s. Part of the change is that scientists have redesigned experiments to counter critics' early objections, such as the possibility that the sleepless learn less because of stress and fatigue, not the loss of sleep-specific brain work. They've also realized that certain kinds of learning seem more linked to sleep than others. Memorizing lists of words or facts--what's called "declarative" memory--doesn't seem all that dependent on sleep. But scientists have lately gathered compelling evidence that people's "how to" learning, or "procedural memory," gets a boost from a bout of sleep.
Last October, for example, Kimberly Fenn and colleagues at the University of Chicago showed sleep's benefits for this type of learning with the help of an annoying, outdated speech synthesizer. When Fenn's computer says "smart," the word comes out sounding more like "smote," and it's hard to figure out what word it's actually saying. "When people first come into the lab, they're really bad," Fenn says, but 30 minutes of training vastly improves their understanding of the garbled speech. What happens over the next 12 hours depends on whether they are allowed to sleep or not. In a series of carefully controlled studies, Fenn showed that while people's learned ability seems to fade away over the course of the day, a night's sleep brings it back. Test-takers who slept showed nearly twice as much improvement as those who did not.
Timing. Similar effects have appeared in other laboratory learning tests of "how to" motor skills (typing a string of numbers as fast as possible) and visual perception (spotting rare diagonal lines in a sea of horizontal and vertical ones). For example, Harvard Medical School's Robert Stickgold trained people to quickly find diagonal lines; then he either let them sleep that night or kept them awake. Both groups got to sleep normally for two nights before being retested, to make sure they weren't fatigued. Only those who got sleep soon after their training showed improvement over their pretraining test scores. "You absolutely have to sleep the first night," says Stickgold.
Other studies hint that in addition to solidifying learned skills, sleep lets the brain forge brand-new strategies that it might never develop while awake. Earlier this year, scientists in Germany described an experiment in which they taught volunteers how to do a math problem. But there was also a shortcut solution to the problem, which wasn't mentioned. After either letting people sleep for eight hours or keeping them up, the researchers found that the sleepers were more than twice as likely to discover the mathematical trick on their own. Jan Born of the University of Lubeck, who led the study, thinks that the sleeping brain may process and repackage learned information so that, upon awaking, the brain can suddenly "see" it in new ways.
How exactly this mental magic happens isn't clear. It does seem that some kinds of learning can take place without long snoozes. Indeed, Stickgold's group found that a 60-to-90-minute nap works as well as a full night's sleep--as long as sleepers get both REM and non-REM sleep. Indeed, recent research indicates that despite the historical interest in dreamy REM sleep, the less sexy deep sleep may also play an essential role in learning.
In one recent study, rats were allowed to explore novel objects like a golf ball mounted on a spring. Then, using tiny electrodes in the rats' brains, the researchers monitored over 100 neurons for several days. Duke University's Sidarta Ribeiro found that electrical patterns recorded as the rats first encountered the objects later got replayed over and over during the next two days, mainly during sleep. The mental reverberation was, surprisingly, most common during the slow-wave sleep. This kind of replay during sleep has also been seen in finches learning new songs and in brain scans of people learning to anticipate a pattern of flashing lights, suggesting that during sleep, the brain goes over new information again and again.
Another way to probe the sleeping brain is to use drugs that interfere with certain chemical changes that occur over the course of sleep. During slow-wave sleep, levels of a chemical called acetylcholine normally drop to low levels, but Born and his colleague Steffen Gais recently tested the effect of this chemical with the help of an Alzheimer's drug that keeps levels artificially high. The scientists gave the drug to half of a group that was taught to memorize a list of word pairs before going to sleep. After a while, everyone woke up and took the test again. People who got the drug did worse, suggesting that the normal chemical changes in slow-wave sleep aid learning.
There are sleep researchers who don't buy any of this. The University of California-Los Angeles's Jerry Siegel, an outspoken critic of the sleep-learning hypothesis, argues that many of these studies simply contradict each other. In the acetylcholine study, for example, he points out that many researchers haven't seen a link between memorizing word lists and sleep. Plus, the drug might just have taken away the normal restful effect of sleep, and fatigued people would do more poorly on any cognitive test. In addition, he notes, people on many common antidepressants get far less REM sleep than normal but don't seem to suffer big learning gaps. And regarding the mathematical insights reported in Born's study, he points out that people get many, many insights during their waking hours, not just during sleep.
Proponents of the sleep-learning theory in the end fall back on a basic fact: Sleep's total isolation from the waking world seems designed to focus attention inward, on information already obtained that needs to get organized. "It's frankly the only biological role for sleep that makes much sense for me," says Stickgold, who notes that the body can rest during quiet wakefulness and that shutting off awareness from the rest of the world is actually a pretty dangerous act that must serve some important function.
Even as debate continues on sleep and learning, new research is showing that sleep is essential not only to brain function but to the proper function of every bodily organ. From a central command post in the hypothalamus, the brain tells us when to go to bed and when to wake up, all the while regulating body temperature, blood pressure, hormone production, digestive secretion, and immune activity. But in addition to the brain's central circadian rhythm control, researchers now know there are at least eight other clock genes in the body's cells that occasionally go off on riffs of their own.
Out of sync. Consider the gut. "When I go to Europe or Japan, my sleep-wake cycle gets back in sync before my digestive system," says Richard Stevens, cancer epidemiologist at the University of Connecticut Health Center. Without getting too graphic about regular morning digestive tract functions, every time-zone crosser can relate to a gut that seems to have its own strong circadian rhythm. In addition to explaining why the first cup of coffee doesn't have the same effect upon landing in Paris that it did the morning before at home, these other clock genes might explain why heart attacks peak in the morning or why the timing of cancer chemotherapy may improve the odds of survival.
Researchers can point to a whole list of diseases connected to sleep deprivation. A 67-year-old man with prostate problems wakes up to go to the bathroom half a dozen times a night, and one morning he has a heart attack. A 19-year-old college student with a diagnosis of bipolar disorder has too much energy to sleep for five nights running and spins into an episode of uncontrolled mania. A healthy man of 30 gets half his required sleep--four hours a night--for a week and ends up in a prediabetic state with the metabolism of his grandfather. An 82-year-old woman falls and breaks her hip, possibly for the same reason that a 42-year-old truck driver slams into a barrier at 3 a.m.--inattention and slowed response because of chronic partial sleep deprivation.
The need for sleep is so strong that without enough of it "people can't even muster enough willpower to stay awake to save their lives," says Carol Everson, who studies sleep in laboratory rodents at the Medical College of Wisconsin in Milwaukee. Inadequate sleep comes with a high cost.
Perhaps the most graphic example of the link between sleep and illness is bipolar disorder, or manic-depression. "I've always believed the mania caused the lack of sleep, and the lack of sleep worsened the mania," says Alexis Maislen, 27, who was diagnosed with the disorder at the age of 19. It is a disease that cycles moods between boundless highs and incapacitating lows. During the highs, she had so much energy that she'd put on her in-line skates and zip around Boston during the wee hours of the morning. She slept maybe a half an hour a night and couldn't talk fast enough to keep up with her crackling ideas. Finally the words came faster than the thoughts behind them. Friends told her she didn't make sense.
Sleep deprivation itself, without underlying illness, won't cause mania. "People [with bipolar disorder] can trigger an episode of mania with sleep dep-rivation. And mania is preceded by an episode of insomnia," says Ruth Benca, a professor of psychiatry at the University of Wisconsin. Once mania is diagnosed, adequate sleep becomes one of the most important parts of treatment.
James Swinney, a retired developer on the board of the Depression and Bipolar Support Alliance, knows it. He's been able to regulate his sleep with drugs, including sleeping pills, though he still can't sleep more than six hours a night and often sleeps only in two-hour snatches. "I make sure I get six hours in a 24-hour period," he says.
The six hours Swinney gets is the best he's been able to do, but it isn't enough. Some rare individuals do fine on six hours or fewer, but most people who claim to need less are kidding themselves. They've simply gotten used to chronic drowsiness the way people get used to constant pain. "Everything we know would suggest that the vast, vast majority of people need more than six hours of sleep a night--somewhere between seven and eight," says David Dinges, chief of the Division of Sleep and Chronobiology at the University of Pennsylvania School of Medicine.
During those seven to eight hours, the cardiovascular system gets a break. "Most people, about 85 percent of those with normal or high blood pressure, have a 20 to 30 percent reduction in blood pressure and a 10 to 20 percent reduction in heart rate during sleep," says William White, professor of medicine at the University of Connecticut Health Center. He has monitored blood pressure for 24 hours in about 5,000 people. "It's amazing when you look at people who sleep like a log. Their blood pressure goes down to about 110 over 70."
But what if you're one of those people whose blood pressure, because of some abnormality, doesn't drop at night? Says White: "There's fairly good evidence that you'll develop more damage to your heart, arteries, and kidneys because your blood pressure is elevated 100 percent of the time, instead of 65 percent of the time for those spending one third of their time sleeping," says White.
Morning misery. With a good night's sleep under their belt, people should be expected to wake up with a heart that's raring to go, right? Yet the incidence of heart attacks and strokes peaks between 6 a.m. and 11 a.m. "We don't know why, but maybe the platelets are more sticky," says Virend Somers, a cardiologist at the Mayo Clinic. And sticky platelets are more likely to clot and clog arteries.
Heart attacks generally don't happen to people with no underlying risk factors, such as hypertension or diabetes. But when those are there, waking up during REM sleep may pose dangers. REM is the one stage of sleep that doesn't lower blood pressure. Indeed, during REM sleep, the sympathetic nervous system gets excited, and sleeping blood pressure and heart rate actually rise. Someone getting up during that dream phase might awaken with a pounding heart. And a fast heartbeat and higher blood pressure coupled with waking activity can put added stress on the heart.
That's a theory anyway. Another possibility for the morning spike is that endothelial cells--the cells that line blood vessels and are important in preventing clots and narrowing of arteries--don't seem to work as well in the morning. Somers measured this in his lab and found just that: The 6 a.m. level is dramatically lower than at other times of day--comparable to what one would expect to see in a smoker or diabetic.
Chest pains and abnormal heart rhythms also tend to occur during REM sleep. "It's not because REM is bad," says Somers. But if people already have an abnormality in their cardiovascular system, the instability of REM sleep may bring on a disaster. Some blood pressure medications can help keep the heart on an even keel during all phases of sleep and into the morning hours.
Those most at risk for heart problems due to bad sleep are people with apnea, a disease that obstructs the airway. The brain responds by signaling for hyperventilation, and the sleeper breathes harder and faster--heard as snoring. "Every time they do that, it causes a surge in blood pressure and heart rate," says White. Blood pressure can shoot up to 240 over 130, and it can happen dozens, even hundreds, of times a night for years. The damage done over time may put people with apnea at higher risk for hypertension, heart failure, and stroke.
When Douglas Bradley studied people with uncontrollable high blood pressure, he found that fully 85 percent of them suffered from sleep apnea. When treated with a mask that forces air into the airway, blood pressure dropped by 10 points on average. The same treatment can also improve heart function in people with congestive heart failure. The National Heart, Lung, and Blood Institute last year included sleep apnea as an identifiable cause of high blood pressure, but the medical community has been slow to embrace the idea.
The field of cancer treatment is even further behind in recognizing, and using, the body's sleep-wake clock to advantage. William Hrushesky published a paper in Science in 1985 on the circadian timing of cancer chemotherapy. In a group of patients with advanced ovarian cancer, women who took their drugs at one time of day had twice as many complications--and were much more likely to die within five years--as women on a different medication schedule.
Do these differences justify altering standard medical practice? Hrushesky has spent his career trying to answer that question, and his research suggests that the answer is yes. Cancer cells don't die when they're supposed to, and they don't stop dividing. "It's hard to even think about cancer without thinking about timing," says Hrushesky. But few oncologists use what is known about treatment timing. "These ideas may be popping, but they're not in vogue," he says.
Someday, scientists may finally figure out what's so special about sleep, for both the workings of the mind and the body. But until they do, experts say the bottom line remains the same: Get your sleep or suffer the consequences. That's why Erck, the health teacher, has sleep specialists visit her classes to talk to students about the many ways that sleep improves the body and the mind, so that they can start to see a good night's rest as a priority. But sleep still has to compete with the call of work, television, video games, and more; and adults and children alike increasingly find it hard to give shut-eye the respect it deserves. "Unfortunately," says Erck, "most kids and adults don't do the right thing to help them get a better night's sleep."