If you're trying to lose weight by cutting calories, make sure to get a good night's sleep. According to a new study, too little sleep appears to hinder your body's ability to burn fat.

Getty Images

By Lynne Peeples

MONDAY, October 4 (Health.com) — If you're trying to lose weight by cutting calories, make sure to get a good night's sleep. According to a new study, too little sleep appears to hinder your body's ability to burn fat.

In the study, 10 overweight but otherwise healthy volunteers on a low-cal diet spent four weeks confined to a research lab. Their diets remained the same throughout, but they slept 8.5 hours a night for two weeks, and just 5.5 hours a night for two weeks.

Although the participants lost roughly the same amount of weight (about 6.5 pounds) on each sleep schedule, the makeup of those pounds differed substantially, the researchers found.

When they were sleep-deprived, the participants lost half as much weight in fat and almost twice as much in muscle and other lean body weight, according to the study, which was published in the Annals of Internal Medicine. Lean muscle is important to maintaining weight loss, as it keeps a person's metabolism running at a healthy clip.

Related links:

The study participants also reported feeling hungrier when they weren't getting enough sleep.

"Losing lean body mass is a side effect, not a benefit, of dieting," says the senior author of the study, Plamen Penev, MD, a professor of endocrinology at the University of Chicago. "It means your body doesn't burn as many calories and requires fewer to maintain the same weight. But cutting even more calories out of the diet may be difficult when you feel hungrier."

Dr. Penev adds, "All these things could conspire to make dieting efforts much less effective at the end of the day."

The study was small, and a laboratory setting in which meals and sleep times are tightly controlled can't be directly compared to the real world, where people can visit the pantry or refrigerator if they're feeling hungry. Still, the study sheds light on the possible interaction between sleep loss and weight gain—both familiar menaces for many Americans.

"It's very difficult to lose weight, so every little bit helps," says David Rapoport, MD, medical director of the sleep disorders center at the New York University School of Medicine. "If you want to take maximal advantage of your diet, and give yourself the best chance of not being forced to cheat on it by hunger, sleep is very important."

Previous research has hinted at the connection between sleep and metabolism, but this study is one of the first detailed looks at what might explain it. One of the advantages of conducting the study in a lab was that it allowed the researchers to closely monitor changes in the volunteers' body fat percentage and appetite hormones.

Dr. Penev and his colleagues were surprised by the stark difference in the losses of body fat associated with each sleep regimen. On average, they found, the dieters lost 3.1 pounds of fat with adequate sleep and just 1.3 pounds with restricted sleep.

They also found that sleep deprivation raised levels of the hormone ghrelin, which stimulates hunger and increases fat retention.

The researchers aren't certain why getting more sleep burns more body fat, but they have some theories. Being awake longer requires more sugar-like substances to maintain the brain's relatively higher activity and metabolism, Dr. Penev notes. This may result in the body utilizing less fat and breaking down more lean body mass (such as protein) to convert into fuel for the brain.

The explanation may be more complicated than that, Dr. Rapoport says. During some parts of sleep, such as rapid eye movement (REM), the brain's metabolism actually rises significantly, he explains.

"It would be an overstatement to say that if you don't get enough sleep, you won't be able to lose weight," Dr. Rapoport says. "But this fits the general scheme that says that when you deprive sleep, you change metabolism."

As the researchers acknowledge, larger and longer studies will be needed to elucidate just how sleep restriction might impede dieting, especially in real-world settings. It's not clear from the study how the body's metabolism may adapt to sleep deprivation over time, for instance.

"The results may or may not generalize to longer-term effects of sleep restriction in the community," says Daniel Kripke, MD, a professor emeritus of psychiatry at the University of California, San Diego. "And they may not apply to people with average sleep durations."

The volunteers averaged 7.7 hours of sleep prior to the study, far more than the 6.5 hours or so reported in the general population, Dr. Kripke notes.