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Stay up late and crawl out of bed too early and you'll get what's coming to you: baggy eyes, a foul mood and -- just maybe -- some extra body fat thrown in for good measure.

Chronic lack of sleep could be one reason people in the United States are getting so broad in the beam, suggest the authors of two recent studies. Their research found that going without sleep seems to elevate blood levels of a key appetite-stimulating hormone, ghrelin. It causes levels of a "stop eating!" hormone, leptin, to take a dive. The likely net effect is an increase in appetite.

"Sleep is not going to be the only answer, but we need to look into it," said Dr. Shahrad Taheri, a lecturer in medicine at the University of Bristol in England, lead author of one of the studies, published late last year in the journal Public Library of Science Medicine.

Taheri says he has firsthand knowledge of the hunger-inducing powers of sleep deficits -- the craving for salty, fatty food and the resulting extra heft that so many new doctors experience when going through their sleep-deprived medical residencies.

In the study conducted by Taheri and colleagues at Stanford University and the University of Wisconsin-Madison, the scientists examined the data from 1,024 volunteers in a long-term sleep study conducted at the Wisconsin campus.

They examined the sleep logs kept by the subjects as well as the duration of their sleep during nights spent at a sleep lab. Analyzing blood samples taken from the subjects, the researchers found a clear pattern. Those who slept the least had the most ghrelin and the least leptin, and for those who slept the longest, vice versa.

The scientists also found that the subjects who got the least sleep had a larger body mass index, a measure of whether someone's overweight or not.

The other study, published in the Annals of Internal Medicine and conducted at the University of Chicago, artificially altered the sleep patterns of 12 healthy men. Each were given two days of restricted sleep (just four hours in bed) or extended sleep (10 hours in bed) on two occasions. All men consumed the same amount of calories during the sleep regimens, and blood samples were regularly drawn.

Again, the scientists found increases in ghrelin and decreases in leptin associated with sleep deprivation. They also found that the volunteers rated themselves as significantly hungrier -- especially for high-calorie foods -- when they had been deprived of sleep.

It does make sense that the body would seek more nourishment if a person is up and about instead of lying flat, said Eve Van Cauter, senior author of the Annals paper and a professor in the department of medicine at the University of Chicago.

But there's a difference between being awake and running around foraging for food, as our forebears did, and being awake and slouching on the couch reaching for the TV remote.

"We think that yes, there is a need for extra energy, but the body is over-reading that need and signaling a need for calories that is way above what the actual requirements are," she said.

The findings, though intriguing, don't prove that getting extra sleep will help shed lard, said Joel Elmquist, an associate professor at Harvard Medical School and Beth Israel Deaconess Medical Center. To show that, carefully controlled trials would be needed in which people seeking to lose weight or prevent weight gain would be assigned to different sleep regimens and the effects of their poundage monitored.

"But a good night's sleep -- that's not a bad thing to recommend," Elmquist said.