MONDAY, June 13 (HealthDay News) -- People who are sleepy by day may be unable to resist calorie-laden comfort foods, new research shows.

The findings, slated for presentation Monday at a meeting of the Associated Professional Sleep Societies in Minneapolis, Minn., add to a growing body of research linking lack of sleep to weight gain and obesity.

In the new study of 12 adults aged 19 to 45, people who were sleepy during the daytime showed decreased activation in the part of their brain that inhibits behavior (prefrontal cortex) when they were shown photos of high-calorie foods such as hamburgers, French fries, pizza, cakes and ice cream, compared with images of low-calorie, healthier foods.

"When you are sleepy, there's a good chance that you won't be able to control how much you eat," said lead study author William Killgore, an assistant professor of psychology at Harvard Medical School and McLean Hospital in Belmont, Mass. "You may be drawn to unhealthy foods because you are not putting the brakes on as well as you would be if you were well-rested."

Researchers looked at activity in the brain's prefrontal cortex via functional MRI (fMRI) scans. Daytime sleepiness was measured using a standardized scale that looks at how often a person dozes off during certain situations such as reading or watching TV.

None of the individuals in the new study had sleep disorders; all participants fell within a normal range of sleepiness. The sleepier they were within this range, however, the less their brain responded when looking at images of fattening foods.

The sleepiest people in the study were also more likely to say they were hungrier than their less-sleepy counterparts.

"Not getting enough sleep is taking its toll on us," Killgore said.

Now, Killgore and colleagues plan to expand the study and see if the results hold up in larger numbers of people. He also wants to determine how sleepiness affects exercise habits.

This study provides another piece of the sleep-weight puzzle, said Dr. Shelby Freedman Harris, director of the Behavioral Sleep Medicine Program at the Sleep-Wake Disorders Center at Montefiore Medical Center in the Bronx, N.Y.

"We already know that when you don't sleep enough, it affects your hormone levels, and now we see how sleepiness can affect your ability to resist high-fat foods," she said.

Studies have shown that inadequate sleep leads to an increase in the hormone ghrelin, which tells you when to eat, and decreases in the hormone leptin, which tells you to stop eating, Harris said.

With insufficient sleep, "you have less leptin and more ghrelin, which tells you to keep on eating , and your prefrontal cortex isn't working as well and can't stop you from overeating," she said.

"Get more sleep," she advised. "Try to go to bed an hour earlier each night, limit caffeine and alcohol, and try to exercise five to six hours before bed," she said. "Sleep is like a dimmer switch; you have to wind down first."

Dr. Louis Aronne, director of the Comprehensive Weight Control Program at New York-Presbyterian Hospital/Weill Cornell Medical Center in New York City, says the findings make sense.

"When you don't get enough sleep, the important hormones change in a direction that would tend to cause weight gain, and this finding shows how it is expressed behaviorally," he said. High calorie, fattening foods provide a sudden, but not sustainable, burst of energy, which is why sleepy people are drawn to them, he explained.

"It is harder to stick with a plan when you don't get enough sleep," he said. Some patients can't comply with the diet, but as soon we fix the sleep, they are able to do well, he said.

Because the study is slated to be presented in a meeting, the findings should be considered preliminary until published in a peer-reviewed journal.

More information

For more on sleep, head to the National Sleep Foundation.