Skimping on sleep may significantly increase the number of calories consumed in a day.

A new study finds that individuals who are sleep deprived may eat up to more than 500 additional calories, compared to someone getting enough sleep. In the small study, presented Wednesday at an American Heart Association meeting, researchers looked at 17 otherwise healthy adults, ages 18 to 40.

For eight nights, half slept a normal amount, while those in the other half were restricted to two-thirds of their typical hours. Both groups ate as much as they wanted and whichever foods they wanted.

On average, the so-called "sleep deprived" group slept an hour and 20 minutes less per night, and consumed an average of nearly 550 additional calories per day.

Though a growing body of research has tied lack of sleep to weight gain, the study did uncover a surprise.

Lack of sleep has been tied to increased levels of leptin -- a hormone that suppresses appetite -- as well as decreasing levels of ghrelin -- a hormone that is thought to trigger hunger. Prior research has suggested the two hormones are regulated, in large part, by sleep patterns.

"How much people eat is a complicated thing, probably governed by many different factors," said Dr. Andrew Calvin, a cardiology fellow and assistant professor of medicine at the Mayo Clinic, as well as a co-investigator on the study.

"The assumption that it's governed by these two hormones is simplistic," he continued. "We were kind of hoping that this would be true ... we could then do something about them to try and treat obesity, but that wasn't the case."

Indeed, the new study concluded that the hormonal shifts were likely a consequence of overeating, rather than a cause.

"It was a surprising finding, that this increased leptin, so they wound up concluding that the ghrelin and leptin changes are not coming from the sleep deprivation, but from the fat," said Max Hirshkowitz, Ph.D., a board member of the National Sleep Foundation and a professor at the Baylor College of Medicine.

"The hormones are probably responding to the feeding behavior," he added.

Hirshkowitz said that previous research has found that sleep deprivation leads to increased calorie consumption because people who are tired are often dysphoric, making them prone to food cravings, particularly high-carbohydrate, high-sugar foods.

Another contributing factor may simply be that being awake for more hours throughout the day means more opportunities to consume calories.

"You can't eat when you're asleep," Hirshkowitz said. (The researchers did not find a significant difference between the number of calories burned between the two groups.)

The two experts said that the new study adds to the growing evidence that sleep deprivation and many sleep disorders are tied to weight gain and raises interesting questions about what, exactly, those connections are. Calvin said there may well be many other hormones at play.

The research highlighted at least one area to focus on in the fight against obesity, which now impacts more than one-third of adults in the U.S.