Lack of sleep changes the circulating levels of the hormones that regulate hunger, boosting appetite and a person's preference for high-calorie, high-carbohydrate foods, University of Chicago researchers report in the Dec. 7 issue of the Annals of Internal Medicine .

Echoing other recent research that has found a link between lack of sleep and the risk of weight gain, the new study is believed to be the first to show that sleep is a major regulator of the hormones leptin, which tells the brain when it doesn't need more food, and ghrelin, which triggers hunger.

"That is the major finding, that we identified the mechanism by which sleep loss affects appetite. The changes in hunger are proportional to the changes in the hormones," said study author Eve Van Cauter, a professor of medicine at the University of Chicago.

For the study, Van Cauter and her colleagues measured the circulating levels of both hormones in 12 healthy men in their 20s. Then they measured the levels after two nights of four hours in bed, with an average sleep time of 3 hours, 53 minutes; and again after two nights of 10 hours in bed, with an average sleep time of 9 hours, 8 minutes.

Then the researchers asked the men to complete questionnaires about their hunger and the desire for different types of food.

When the subjects slept only four hours nightly, leptin levels decreased by 18 percent and ghrelin levels increased 28 percent. What's more, their reported hunger increased by 24 percent, and they craved calorie-packed foods with high carbohydrate content, such as candy, cookies and cake, the researchers said.

Sleep deprivation is a fact of life for many Americans. Sleep duration among U.S. adults has decreased by up to two hours a night since the 1960s, according to the National Sleep Foundation. In 2002, more than 37 percent of young adults said they slept less than seven hours a night, compared to about 16 percent in 1960.

Another expert who has studied the effect of sleep loss on weight said the new study adds to the growing body of research on the subject.

"This, to my knowledge, is the first real experimental study of sleep deprivation on food intake and regulatory hormones," said Dr. Steven B. Heymsfield, a professor of surgery at Columbia University and St. Luke's-Roosevelt Hospital Center in New York City.

While the study had a small number of participants and wasn't lengthy, it was "very well-designed and controlled," Heymsfield said.

In a recent study, Heymsfield found that people who slept four hours or less a night were 73 percent more likely to be obese than those who slept seven to nine hours nightly.

Why a lack of sleep affects the hunger hormone levels isn't known, Van Cauter said. Some studies have suggested that sleep loss is tied to an increase in sympathetic nervous system activity, which can inhibit the release of leptin, the hormone that tells you to stop eating.

For future research, Van Cauter said she wants to investigate whether sleep duration affects how well people adhere to a diet, and whether the success of a weight-loss plan is based on the amount of sleep a person gets. She said she also wants to study whether some people might be immune to sleep loss and an increase in appetite.

Meanwhile, her advice: "Know how much sleep you need, know when you are running a sleep debt, and pay it."

Heymsfield added: "If you have a weight problem to begin with, watch out for periods of sleep deprivation. You will be vulnerable to weight gain."

SOURCES: Eve Van Cauter, Ph.D, professor, medicine, University of Chicago; Steven B. Heymsfield, M.D., professor, surgery, St. Luke's-Roosevelt Hospital Center and Columbia University, New York City; Dec. 7, 2004, Annals of Internal Medicine