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Subjects tended to eat large amounts of fatty foods from 10PM-4AM

Night owls are more likely to gain weight than people who get good sleep because they tend to graze the kitchen for junk food in the wee hours of the morning, a new study suggests.

Researchers at the University of Pennsylvania found that people who were kept up until 4 a.m. in a sleep lab ate more than 550 additional calories during the late-night hours.

"People consumed a substantial amount of calories during those late-night hours when they would normally be in bed," said study author Andrea Spaeth, a doctoral candidate in the psychology department at the University of Pennsylvania. "Those calories also were higher in fat compared to the calories consumed at other times of day."

As a result, subjects kept up late gained more weight during five days of sleep deprivation than people in a control group who were allowed to get good sleep, Spaeth said.

Late-night overeating is likely the result of hormonal changes that occur in people who are sleep-deprived, said Dr. W. Christopher Winter, medical director of the Martha Jefferson Sleep Medicine Center in Charlottesville, Va.

They tend to experience an increase in their levels of ghrelin, a hormone that stimulates hunger cravings, and a decrease in levels of leptin, a hormone that makes people feel full.

"Now you're in a situation where you are craving bad food and more of it, and your body feels less full when it gets that bad food," Winter said.

The research team monitored the eating habits of about 200 people who, for five days straight, were kept up until 4 a.m. and then allowed only four hours of sleep. They remained in the lab the whole time, going through in groups of four or five at a time.

Subjects were allowed to eat whenever they liked, and trained monitors in the sleep lab maintained a running tally of the amount consumed and the times at which they ate.

Researchers then compared their calorie intake and weight gain to that of a control group allowed a good night's sleep in the same lab with the same food availability. "The only difference between the two groups was sleep," Spaeth said.

"They lived in a suite, and in the suite there was a kitchen with a fridge and microwave."

The eating habits of the control group remained unchanged. The sleep-deprived group began eating additional calories between 10 p.m. and 4 a.m., and they tended to eat fattier foods during that time period. "That does kind of mimic the real world, when you're up late at night and you drift over to your fridge," Spaeth said.

There was one key difference between the lab and the real world. Since the study took place in a hospital, the suite's kitchen was stocked with hospital food. "I'm wondering if the effect would be stronger in the real world, where you have access to more calorically dense foods," Spaeth said.

The findings are published in the July issue of the journal Sleep.

Previous studies have shown a link between inadequate sleep and weight gain, Winter said, but this research is valuable because it provides precise observations in a laboratory setting.

"Anytime you're dealing with studies in the field, you're often relying on some sort of food diary or the recall of the patient," Winter said. "It's amazing how much food a person can eat and not remember. It's hard to keep track of people in terms of their eating and in terms of their sleep. When people are in a lab, they can perfectly control conditions and report them."

Blacks gained more weight than whites, and males gained more weight than females. Researchers currently are undertaking follow-up research involving detailed calorie counting to try to explain these differences, Spaeth said.

The study adds more weight to the growing mound of evidence suggesting that people who want to control their weight need to get seven to eight hours' sleep a night, Winter said.

People who can't get a good night's sleep -- such as people who are traveling or working late to meet a deadline -- need to pay extra attention to their food cravings, he added.

"God knows I'm aware of it," Winter said. "I'm usually traveling a lot, and I can feel myself craving food I don't really need and didn't know I wanted. I know when I'm going through an airport late at night and I see chocolate-covered pretzels, I know I'm not craving them because I'm hungry. I think if patients are more aware of those things, they're going to say, 'I'm not going to eat this because I'm not hungry, and it's not going to do me any good.'"

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