Yawning at your desk and feeling a bit bloated? The two sensations could be connected. A wealth of studies are making a connection between sleepless nights and extra pounds.

Most recently, German and Swedish researchers on Tuesday presented data at a meeting of the Society for the Study of Ingestive Behavior, showing that even short-term sleep deprivation altered study subjects' appetites. The study has not yet been published in a peer-reviewed journal.

Sleep-deprived subjects felt hungrier and also had higher levels of ghrelin, a hormone produced by the stomach that stimulates appetite. A full night of sleep deprivation also reduced the amount of energy the subjects used at rest, suggesting that insomnia makes it harder to lose weight as the body holds on to calories.

When we are sleep deprived we are likely to eat more calories because we are hungrier ... sleep loss also means we burn off fewer calories, which adds to the risk of gaining weight, the researchers said in a statement Tuesday.

In a paper published in the American Journal of Human Biology in April, University of Chicago researcher Kristen Knutson reviewed evidence from a number of sleep studies and found an association between getting less than six hours of sleep and increased body-mass index or obesity.

Sleep deprivation not only impacts the hunger-producing hormone ghrelin, but it may also drive down levels of leptin, ghrelin's counterpart, which produces the full feeling of satiation.

It's estimated that 18% of people in the U.S. get less than six hours of sleep a night. That translates into about 53 million people that could be feeling the weighty effects of sleep deprivation. Knutson's review also found that the association between sleep deprivation and obesity risk is greater in children, adolescents and poorer people than in other groups.

Poor sleeping patterns are not random, and it is important to consider the social, cultural and environmental factors that can cause inadequate sleep so that at-risk groups can be identified, Knutson said in a statement.

Lack of sleep may also trigger genes associated with metabolism and appetite to contribute to weight gain, according to recent research from a University of Washington-led team.

The scientists in this study looked at self-reported height, weight and sleep habits in more than 1,000 pairs of twins to ascertain how sleeping patterns and genes interact with respect to weight gain. (Twin studies are helpful in separating genetic and environmental contributions to disease, since the DNA of identical twins is nearly a perfect match. Fraternal twins, which are about 50% identical genetically, are also useful since they were raised in the same environment.)

The results, published in the journal Sleep in May, found that in twins sleeping less than seven hours, differences in body-mass index were at least 70% attributable to certain genetic factors. Meanwhile, in twins that averaged nine or more hours of sleep, genetic factors were only 32% responsible for weight variations, and environment was much more of a factor.

The results suggest that shorter sleep provides a more permissive environment for the expression of an obesity-related gene. Or it may be that extended sleep is protective by suppressing expression of obesity genes, author Nathaniel Watson said in a statement.

While you probably take these studies as an excuse to trade your running shoes for a nightcap, getting at least eight hours of sleep seems to be a key component of maintaining a healthy weight, along with diet and exercise.