As bleary-eyed college students in exam week will attest, lack of sleep impairs mood, performance, and judgment. They might guess, however, that the fast food and candy gobbled down during an all-nighter are far worse for bodily health than are the lost hours of slumber.
After all, scientists have long been preaching that too many Big Macs and too few workouts are bad for you, but they have yet to demonstrate any definitive health costs of chronic sleep loss.
Bolstered by new evidence, however, some scientists are suggesting that poor sleep habits are as important as poor nutrition and physical inactivity in the development of chronic illness.
They say that this country's sleep debt may be contributing to its current epidemics of obesity, diabetes, and cardiovascular disease.
People in the United States sleep an average of 7.0 hours on weeknights, 1.5 hours less than they did a century ago, according to the National Sleep Foundation in Washington, D.C. One-third of the population sleeps 6.5 or fewer hours, far less than the 8 hours that many sleep specialists recommend.
Several recent studies report that reducing sleep to 6.5 or fewer hours for successive nights causes potentially harmful metabolic, hormonal, and immune changes, at least in test volunteers in the sleep lab.
"All of the changes are what you find in normal aging," says sleep researcher Eve Van Cauter of the University of Chicago.
It's still too early for doctors to start prescribing sleep to ward off age-related disease. Scientists agree that the findings are preliminary and that larger experiments are needed. If the story bears out, however, U.S. sleep habits may be having enormous public health consequences.
Early sleep-loss research focused on military personnel, rescue workers, shift workers, and others for whom on-the-job wakefulness is crucial. These studies examined the performance declines that occur with extended periods of total sleep loss.
Investigating the relationship between health and the pattern of partial sleep deprivation that the average American faces is a much newer research endeavor. Van Cauter and her colleagues helped launch the field with a surprising 1999 study that showed sleep deficits of several hours a night can impair the body's processing of the sugar glucose.
The study reported that 11 healthy, lean young men showed signs of insulin resistance after several nights of sleep restriction. Insulin resistance, a condition in which the body handles glucose poorly because cells respond inefficiently to insulin, is a precursor to type II diabetes.
Experimenters carefully controlled the mens' sleep time, food intake, and exercise during their two-week stay in the sleep lab. During the sleep-restriction phase of the experiment, the men were kept awake by minimally stressful activities, such as watching television, joking with the staff, and playing games, Van Canter says.
The restraints imposed in the study, which permitted only 4 hours of sleep a night for six nights, were more severe than most people in the United States experience. The study also excluded women.