Scientists have found that sleep deprivation increases levels of a hunger hormone and decreases levels of a hormone that makes you feel full. The effects may lead to overeating and weight gain.

It could explain why so many Americans who are chronically sleep-deprived also are overweight. And it could be part of the reason sleepy college students, new parents and shift workers pack on pounds.

Researchers say getting enough shut-eye might be a critical component of weight control. And nutritionists one day might routinely advise dieters to "sleep it off" as well as to cut calories and increase exercise.

"We know the obesity epidemic is due to overeating — too big portions, too much rich food and too little activity — but why do we crave too much of these rich foods?" says Eve Van Cauter, a University of Chicago sleep researcher who is the lead investigator on one of the new studies. Maybe, she says, it's because "we are sleep-deprived and unable to curb our appetites."

Obesity experts are intrigued by these findings. "We've known that people use food as a pick-me-up when they are tired, but now it appears they are hungrier than we realized, and there is a hormonal basis for their eating," says Thomas Wadden, director of the Weight and Eating Disorders Program at the University of Pennsylvania in Philadelphia.

"The one thing that is clear is that during the holidays people should sleep as much as they can and not get too stressed out. That could prevent some of the holiday weight gain," Wadden says.

Samuel Klein, director of the Center for Human Nutrition at Washington University School of Medicine in St. Louis, says it looks like "we should tell overweight patients to get more sleep. This is advice that's easier to follow than eat less and exercise more.

"It means watching a little less TV at night and getting to bed earlier," Klein says, "Who on earth would argue about getting more sleep?"

Getting enough sleep and controlling stress are subtle things that could have an impact on weight, agrees Louis Aronne, president of the North American Association for the Study of Obesity and director of the Comprehensive Weight Control Program at New York Presbyterian Hospital.

Van Cauter believes some people may be extra sensitive to sleep deprivation, which "makes it very hard for them to control their appetites. Our body is not wired for sleep deprivation," she says. "The human animal is the only one that does this."

Sleep does indeed appear to be an important piece of the weight-control puzzle, says Stanford University sleep researcher Emmanuel Mignot, who also is releasing new research.

If that's true, it might be part of the solution to the nation's obesity problem. Sixty-five percent of Americans are overweight or obese, which increases their risk of heart disease, type 2 diabetes, cancer and other diseases.

This percentage takes on a special significance when balanced against the fact that an estimated 63 percent of American adults do not get the recommended eight hours of sleep a night, according to the National Sleep Foundation. In fact, the average adult gets 6.9 hours of sleep on weeknights and 7.5 hours on weekends, for a daily average of seven hours.

Van Cauter has spent 25 years doing research on the hormones that are affected by sleep. She says sleep deprivation activates a small part of the hypothalamus, the region of the brain that also is involved in appetite regulation. She is especially intrigued by, and has done several studies on, two critical hormones involved in regulating food intake: ghrelin and leptin.

They influence eating in different ways. Ghrelin is an appetite-stimulating hormone released mostly by the stomach. When ghrelin levels are up, people feel hungry, Van Cauter says. On the other hand, leptin, considered a satiety or fullness hormone, is released by the fat cells and tells the brain about the current energy balance of the body.

When leptin levels are high, that sends a message to the brain that the body has enough food, and the person feels full, she says. Low levels indicate starvation and increase appetite.

The hormones "have been called the yin and yang of hunger," Van Cauter says. "One is the accelerator for eating (ghrelin), and the other is the brake (leptin)."

Van Cauter, who directs the Research Laboratory on Sleep, Chronobiology and Neuroendocrinology at the University of Chicago School of Medicine, examined the effect of sleep deprivation on these two hormones for her latest study, published in Tuesday's Annals of Internal Medicine. She had 12 healthy, normal-weight men, average age 22, come into a hospital laboratory to sleep and eat dinner and breakfast.

On one occasion, they were limited to four hours in bed for each of two consecutive nights. At another time, they were allowed up to 10 hours in bed for two nights. Their blood was drawn at regular intervals, and they were asked about their hunger. Findings: