Getting too little sleep might prevent dieters from losing as much body fat as they otherwise would have, a small study suggests.

The findings, published Monday in the Annals of Internal Medicine, add to evidence that sleep habits play a role in weight regulation. They also suggest that people embarking on a weight-loss plan may want to make sure they are catching enough shut-eye each night, researchers say.

The study included 10 overweight men and women who lived in a sleep lab for two separate two-week periods. During both, they were kept on the same calorie-restricted diet; but for one period, the participants slept for 8.5 hours per night, while during the other, they got 5.5 hours.

Researchers found that the dieters lost the same amount of weight under both conditions -- just under 7 pounds, on average. But during the sleep-restricted period, they mainly lost muscle rather than fat.

When participants got 8.5 hours of sleep, more than half of their weight loss came from shedding fat; when they got 5.5 of sleep, only one-quarter of their weight loss came from fat -- translating to a 55 percent reduction in fat loss.

Instead, the majority of people's weight loss during the sleep-restricted period came from lean body tissue, which refers to muscle and any other body tissue that is not fat.

So they lost the same amount of weight, but the composition was different, said senior researcher Dr. Plamen Penev, an assistant professor of medicine at the University of Chicago.

Successful dieters always shed a certain amount of muscle, Penev noted in an interview, but ideally one wants to limit that loss in favor of shedding excess body fat. Insufficient sleep, the current findings suggest, might interfere with that.

The study has a number of limitations. Besides its small size, it also looked only at short-term weight loss. More research is needed to see how sleep duration might affect dieters' body composition over time, Penev said.

It's also unclear how well these findings from a tightly controlled sleep-lab setting might translate to the real world, according to Penev.

Still, the findings do add to a body of research linking sleep habits to body weight. A number of studies have found that self-described short sleepers -- typically defined as those who get less than 6 hours of sleep each night -- tend to weigh more or gain more weight over time than people who get seven to eight hours of sleep per night.

Those studies do not, however, prove that sleep differences are the reason for the weight differences. Small sleep-lab studies such as the current one help researchers zero in on the effects of sleep itself.

Lab studies have suggested, for example, that sleep loss may alter people's levels of the hunger hormones leptin and ghrelin. Leptin is secreted by fat cells; low blood levels of the hormone promote hunger, while increases tell the brain that body is full and encourage calorie burning. Ghrelin is secreted by the stomach to boost appetite.

In their study, Penev and his colleagues found that under the sleep-restricted condition, participants reported greater hunger during the day compared with the 8-hour sleep condition -- even though they consumed the same number of calories during both periods. They also had higher blood levels of acylated ghrelin, one form of the appetite-boosting hormone.

This, Penev said, raises the question of whether, outside the tight control of the lab, the sleep-deprived dieters would have eaten more.

The study suggests that if you are trying to lose weight by restricting your calories, it may be more difficult if you are sleep deprived, Dr. Shahrad Taheri, of the University of Birmingham in the UK, told Reuters Health in an email.

As for how much sleep is enough, I don't think at this stage we can recommend a specific number of hours of sleep, as sleep is very individualized, said Taheri, who co-authored an editorial published with the study.

But what we can do, Taheri added, is pay more attention to our daily routines of eating, physical activity, and also sleep.

Penev agreed that there is no one-size-fits-all prescription for sleep. He suggested that people try to notice how much sleep they generally need to feel refreshed the next morning; for some people that may be 6 hours, for others it may be 8.

Both Penev and Taheri said that more studies are needed in real-world settings. According to Penev, a study might, for instance, follow patients at a weight-loss clinic to see how their typical sleep habits correlate with their weight-loss success.

As for why fewer hours in bed may cause the body to preferentially lose muscle over fat, Penev said he and his colleagues can only theorize. Their guess is that the extra waking hours increase the need for glucose (sugar) in the brain and other parts of the nervous system -- and they get that extra fuel from breaking down muscle.