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November 17, 2005

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Clifford Saper believes that people who lose small amounts of sleep on a daily basis can suffer impaired mental performance and an increased risk for heart disease.

Waking up to how we sleep and dream

Do people learn while they sleep?

By William J. Cromie
Harvard News Office

We spend about a third of our lives asleep. What really goes on during this time?

The answer: more than anyone ever dreamed. The more researchers study sleep and dreams, the more complicated things turn out to be. A simple answer like "We sleep to rest our bodies and brains" just doesn't account for all the things researchers are discovering.

The Oct. 27 issue of the prestigious science journal Nature devotes almost 40 pages to bringing readers up-to-date on what happens during sleep. Three of the articles are by Harvard Medical School scientists who discuss such things as an on-off sleep switch, and learning while we sleep.

This research is based on well-established findings that the brain doesn't stop working when we sleep. During as much as 20 percent of our sleeping time, we exhibit rapid bursts of eye movements, and our brains are almost as active as when we are awake. Called REM (rapid eye movement) sleep, these are periods of vivid dreaming. During the rest of our sleep, even though consciousness is greatly diminished, our brain cells remain surprisingly active.

Robert Stickgold's research reveals that people learning to play piano scales or to memorize word lists show improvement after a good night's sleep but not after an equivalent period of being awake. (Staff photos Kris Snibbe/Harvard News Office)

Clifford Saper, James Jackson Putnam Professor of Neurology and Neuroscience, and his colleagues at Beth Israel Deaconess Medical Center study key nerve circuits that switch us from waking to sleeping and back. Two small clusters of nerve cells in the hypothalamus, a cherry-size area behind the eyes, shut down our arousal circuits when we sleep. The switch is turned back on by the time of day and the length of time spent awake before going to bed.

Normally, switching is abrupt. We fall asleep or wake up quickly. Another group of nerve brain cells stabilizes the switch to prevent it from going off at the wrong time, and causing us to fall asleep while, say, driving at night. People who lack this stabilizer suffer a condition known as narcolepsy. Narcoleptics suddenly drop off to sleep at inappropriate times during the day even after they enjoy a good night's sleep. Fortunately, narcolepsy is relatively rare.

"Our findings help explain how various drugs affect sleep-wake cycles, and they provide the basis for a wide range of environmental influences that shape those cycles into optimal patterns of survival," Saper notes. "Learning to control our wake-sleep system holds the promise of improved health and cognitive performance." As an example, people who lose small amounts of sleep on a daily basis in our fast-moving society can suffer impairment of mental performance and an elevation of risk for heart disease.

"Because older individuals sleep about half an hour less per day," Saper continues, "it's possible that at least some of their cognitive decline and increase in cardiovascular disease might be explained by sleep restriction. Similarly, sleep loss might impair performance among adolescents who arise early for school, shift workers, overnight long-haul truckers, and medical personnel working long shifts in hospitals."

Why we sleep and dream

The basic question of why we sleep remains unanswered. "I think we sleep because of the basic geometry and biology of nerve cells," Saper offers. Learning occurs in all nerve cells by the transfer of chemical signals at the junctions between these cells. That process can take a long time, as much as hours, because of geometry and distance. "Thus, brain cells, after collecting new information in the form of biochemical changes during the wake cycle, need some downtime to fix those changes," that is, to strengthen new connections between cells that are the basis of memory and learning. So as the body rests, the brain keeps working.

Dreaming, many scientists believe, is just a meaningless byproduct of this memory consolidation. The characters and activities that show up on the screen in the little theater of the mind actually make no sense, but the irresistible human urge to organize these images prompts us to put together a story, however fragmentary, and to try to give our dreams meaning.

sleeping woman illustration
Staff illustration Georgia Bellas/Harvard News Office

"Studies show that hallucinatory mental content is lowest during active waking and highest during REM sleep," says Allan Hobson, a professor of psychiatry at Harvard Medical School. "The incidence of thinking is highest during quiet waking and lowest during REM sleep. The implication of these findings is that the sleeping brain can either generate its own perceptions or it can think about them. It cannot do both at the same time. Therefore, dreaming is as hallucinatory and thoughtless (delusional) as so-called mental illness."

Think of that next time you try to make sense out of your dreams.

Sleeping on it

Robert Stickgold, an associate professor of psychiatry at Harvard, picks up on the subject of memory consolidation, or the idea of sleeping on a problem. He and his colleagues conducted a number of experiments demonstrating that a good night's sleep triggers changes in the brain that improve memory.

For example, tasks such as playing piano scales or memorizing a list of words reveal that people "show improvements after a night's sleep but not during an equivalent period of being awake," Stickgold reports. "Total or partial sleep deprivation can prevent this normal overnight improvement."

"MRI [magnetic resonance imaging] scans show us that, during sleep, brain regions shift dramatically," notes Matthew Walker, director of the Sleep and Neuroimaging Lab at Beth Israel Deaconess Medical Center. "During sleep, it seems as though you are shifting memory to more efficient storage regions within the brain. Consequently, when you awaken, memory tasks can be performed more quickly and with less stress and anxiety."

Such research may help explain why children, teenagers, and especially, infants need more sleep than adults. "At 12 months of age, infants have an immense amount of new material to consolidate and, consequently, this intensive period of learning may demand a great deal of sleep," Walker says.

These new revelations may prove important to patients who suffer brain injuries. A good example includes stroke survivors who must relearn to move their arms and legs and to speak clearly again.

Strong evidence exists that some types of motor-skill or procedural learning is consolidated during sleep, but evidence is more modest for declarative memories, such as what you had for dinner last Tuesday or the capital of South Dakota. "When sleep learning or consolidation does occurs, we don't have a good understanding of when it takes place, in terms of which sleep stages and times of night," Stickgold admits. "And finally, why these processes are preferentially or exclusively restricted to sleep is completely unknown."

Related stories:

  • The Big Picture
    Matthew Walker, sleep researcher

  • Just sleep on it
    And empty the brain's 'in box'

  • Copyright 2007 by the President and Fellows of Harvard College