When it comes to losing weight, the amount of shut-eye a person is getting might be just as important as the amount of food they're eating.

According to new research being published in Annals of Internal Medicine, a lack of sleep can hinder a dieter's ability to shed excess body fat, with even short periods of sleep deprivation undermining weight-loss efforts.

For the study, researchers at the University of Chicago put 10 overweight but otherwise healthy adults who were on a moderately calorie-restricted diet. The group was randomly assigned to sleep either 5.5 hours or 8.5 hours each night in a closed clinical research environment.

The two groups then switched so that the sleep-deprived started getting more sleep, and the well-rested got less for two more weeks.

Researchers measured loss of fat and lean body mass. The participants lost about 6.6 pounds during each two-week intervention.

During the two weeks where they got adequate sleep, men and women lost 3.1 pounds of fat and 3.3 pounds of fat-free body mass (which was mostly made up of muscle). By contrast, men and women lost 1.3 pounds of fat and 5.3 pounds of fat-free mass when they slept for shorter amounts of time, the researchers found.

"These results highlight the importance of adequate sleep for maintenance of fat-free body mass when dieting to lose weight," said lead author Dr. Plamen Penev, an assistant professor of endocrinology.

The researchers then investigated why sleep-deprived dieters didn't do as well as those getting the shut-eye they needed.

They measured levels of hormones in both groups and found that sleep restriction caused an increase in levels of ghrelin, a key hormone that stimulates hunger and promotes retention of fat.

Participants saw close to a 10-point increase in their levels of ghrelin during the two weeks when their sleep was restricted, rising from 75 nanograms per liter of blood (ng/L) to 84 ng/L, the study showed.

The study participants trying to get by on less sleep also reported to researchers that they felt hungrier.

"This could explain why sleep-deprived participants also reported feeling hungrier during the study," Penev said in a statement.

Dr. Charles Samuels, the medical director of the Centre for Sleep and Human Performance in Calgary, is not surprised by the findings, noting that sleep deprivation can make us crave high-calorie foods.

"When we have less sleep, our appetite goes up for calorie-dense foods, like fats and carbohydrates, that of course cause us to gain weight. But also our body's neuroendocrine factors -- or hormones -- don't operate appropriately so that we can keep our weight down," he says.

Samuels suggests that sleep may be that "X factor" that explains why some people can lose weight and others can't. And yet, he says, many people take sleep for granted

"We pay no attention to sleep. It is the one thing we give up to get on with our life and do what we want to do, and it is actually quite important," he tells CTV News.

Physiologist Jean-Pierre Chaput, a researcher at the Children's Hospital of Eastern Ontario, has also looked at the link between sleep and weight, and says this latest study shows that there's more to losing weight than adjusting the diet.

"Based on my studies, we found short sleep duration, was the best predictor for overweight and obesity in adults and children -- more a predictor of weight gain more than diets or exercise," he says.

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip