Scientific studies show that sleep deprivation can cause weight gain.

Take this pill. Try this diet. Buy this machine. Amid the steady stream of marketing hype designed to capitalize on our collective fear of fat, it's a rare thrill to hear about weight loss news that's backed by actual science.

But are studies always trustworthy? We investigated recent weight loss claims straight out of the lab and found the answers to be more complex than you might think.

The Finding: "Sleep more, weigh less."

Behind the science: If you want to diet successfully, go to bed. That's one message of the National Sleep Foundation (NSF), a nonprofit organization that receives about half its funding—$1.5 million a year—from pharmaceutical companies like Sanofi-aventis, which markets the sleeping pill Ambien, and Sepracor, maker of Lunesta.

Given the sources of NSF's income, one might question its statement that sleeplessness is "the royal route to obesity." But in this case, the rhetoric is rooted in several good studies that are not funded by the foundation or pharmaceutical companies.

You don't need a PhD to see why, on four hours of sleep, one would be less likely to work out and more apt to reach for a box of doughnuts to get through the day than on eight hours of good rest. Now, however, scientists have more details about how sleep affects hunger. "If you limit the amount of time you are in deep sleep, the brain interprets that as an insufficiency of energy stores," says Jana Klauer, MD, a research fellow at the New York Obesity Research Center of St. Luke's—Roosevelt Hospital. "It thinks you are in a state of starvation"—and ups your appetite accordingly.

In the past 15 months, two studies published in the journals Annals of Internal Medicine and PLoS Medicine have revealed the chemical mechanisms that throw a sleep-deprived person into a calorie-craving panic. When you don't get a full night's rest, the body's fat cells secrete up to 18 percent less of the hormone leptin, which signals to the brain that you have sufficient energy reserves and therefore don't need food. It's like the fuel gauge on your car: If your leptin levels are low, your brain wants to refill the body's tank. At the same time, the stomach produces too much ghrelin, a hormone that stimulates appetite.

Among 1,024 volunteers, ghrelin levels rose 15 percent in those who slept five rather than eight hours a night. Not surprisingly, the shortest sleepers were also the chunkiest.

Unfortunately, even with the best of science, no one has delivered a cure-all. Getting eight hours of sleep a night might make it easier to lose weight, but you still have to do the work by eating less and exercising more. And that's true for all diet breakthroughs at this point. "It's the food in and the calories out," Buxton says. "The rest stays around the middle."

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