A new study in the Annals of Internal Medicine offers some advice a lot of dieters can embrace: Sleep in. It is a little more complicated than that, of course, but the University of Chicago's Plamen Penev and his team found that getting a full night's rest is important for burning fat, not just losing weight. (Watch a report about the study.) Here's a rundown of the findings:

What were the researchers studying?
The effects of sleep and sleep deprivation on weight loss. Dr. Penev and his associates kept 10 overweight adults in a sleep lab for four weeks, feeding all of them the same restricted-calorie diet — 90 percent of the calories they needed — but changing the amount of sleep they got. Each participant spent two weeks with 8.5 hours to sleep (real sleep time: 7.5 hours) and two weeks with 5.5 hours (real sleep time: 5.3 hours).

What did they find?
There was no difference in weight loss: All participants lost about seven pounds in each two-week phase. But during the full-night phase, half of that lost weight was fat; during the short sleep cycle, less than a quarter of the lost weight was fat, and the rest was lean body mass, including muscle and solid organs — things you don't want to lose. "If your goal is to lose fat, skipping sleep is like poking sticks in your bicycle wheels," says Penev.

The researchers speculate that it has something to do with the "hunger hormone" ghrelin, which the stomach produces to stimulate appetite. The sleep-deprived group both felt hungrier during the day and had higher levels of ghrelin. Penev also suspects that lean body mass is the body's preferred fuel when we're awake and active, so more waking hours means more muscle, and less fat, being burned off.

It was a small study. Does it really tell us anything?
Penev's team says that more research is needed, especially in "real world" conditions. Besides size, another limitation is that the study only measured short-term weight loss. Also, researchers still don't understand the sleep-weight equation very well: Why, for example, do rodent studies show that less sleep increases weight loss, but human studies consistently show the opposite? On the other hand, Penev says, there's a good chance his study underplayed the benefits of sleep, since the uniform diet kept hungrier short-sleepers from eating more. 

What's the bottom line?
"Put aside the work, or that late-night TV show, and get some shut-eye," says Amina Khan in the Los Angeles Times. "Compared to exercise and diet, it's the easiest part of healthy weight loss." Even if Penev is wrong about the fat-burning benefits of sleep, says Katherine Hobson in The Wall Street Journal, "whether or not you're trying to lose weight, there's no real downside to making sure you're snoozing enough."

Sources: LA Times, Reuters, New York Times, Daily Telegraph, Wall Street Journal