Getting too little sleep might prevent dieters from losing body fat, according to a small U.S. study.
The study, published in the Annals of Internal Medicine, adds to evidence that sleep habits play a role in weight regulation and suggest people embarking on a weight-loss plan may want to make sure they are catching enough shut-eye each night.
The study included 10 overweight men and women who lived in a sleep lab for two separate two-week periods.
During both periods they followed 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 from the University of Chicago found 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.
But when they got 5.5 hours of sleep, only one-quarter of their weight loss came from fat, translating to a 55 percent reduction in fat loss. The majority of their weight loss 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 researcher Dr. Plamen Penev, an assistant professor of medicine at the University of Chicago. Penev said successful dieters always shed a certain amount of muscle but wanted to limit that loss in favor of shedding fat.
The study, however, 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.
Penev said it was also unclear how well these findings from a tightly controlled sleep-lab setting might fit the "real world."
A number of studies have found 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 more sleep.
Lab studies have suggested 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.
Penev's study found that under 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.
Penev said there wsa no one-size-fits-all prescription for sleep and more studies are needed in real-world settings.