The Oct. 27, 2005 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.
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.”