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Did you know losing sleep leads to weight gain? Integrative physiology associate professor Kenneth Wright Jr.’s research reveals why.

If you’re staying up late to prepare that sales report for your soulless boss or finish a spicy page-turner, you may want to shop the latest sale for bigger pants — you’ll need them.

Yes, researchers have known for a while about a link between sleep and weight. People who sleep less than six hours are at greater risk for obesity, which can lead to diabetes, heart disease and even cancer.

Associate professor of integrative physiology Kenneth Wright Jr., director of ŷ���ڱ���Ƶ-Boulder’s sleep and chronobiology lab, wanted to dig deeper into how losing sleep leads to weight gain. The importance of sleep for our health has been a question since his days as an undergraduate at the University of Arizona two decades ago.

“I had a professor who studied it, and it was fascinating,” Wright says. “It was such an unknown back then, and I thought it would be good to contribute not only to the scientific community but to the general public.”

Wright describes looking at weight gain in terms of energy your body uses and food intake. When they’re equal we maintain a stable weight, but an increase or decrease of either leads to changes. He wanted to see what was happening to both sides of the equation related to sleep.

Accordingly Wright and his team stuck 16 people — eight men and eight women — into the lab and meticulously tracked their sleep, metabolism and eating habits for two weeks. Researchers kept an eagle eye on everything — calculating subjects’ metabolism by measuring how much oxygen they used and carbon dioxide they produced, recording every bite of food and imposing strict bedtimes.

During that first week half the people slept up to nine hours and the other half just five. Besides three scheduled meals and two snacks, additional snacks were readily available all the time: potato chips, red or green grapes, cookies, cereal bars, pretzels, peanut butter crackers, roasted or salted cashews, diet or regular soda, whole and skim milk, yogurt, orange juice and everyone’s favorite, vanilla ice cream.

For the second week, the tables were turned: the nine-hour sleepers slept five hours and the sleep-deprived slept nine hours.

The findings? Losing just a few hours of sleep several nights in a row can lead to weight gain almost immediately.

But at second glance the results weren’t that clear. Wright found that sleep-deprived metabolism fires up a like a furnace. In fact, those with five hours of sleep burned an extra 111 calories a day. But here’s the catch. You’re obviously more active when you’re awake and burning calories. But when you lose sleep you overeat.

“The light sleepers ended up eating a lot more than those who got nine hours of sleep,” he says.

And by the end of the first week the sleep-deprived subjects had gained an average of two pounds. One sleepy guy ate a half gallon of vanilla ice cream in one sitting.

During the second week, sure enough, those who had slept nine hours put on weight when they got just five hours of shut-eye, and the other group began to lose the weight they put on in their first sleep-deprived week.

Wright says if you skimp on sleep you’ll not only eat more, but you’ll choose more junky foods, and you’ll eat them later in the day — a trifecta of future fat.

“Those with less sleep ate more carbohydrates and when they ate also changed,” he says. “They ate a smaller breakfast and a lot more after dinner. Actually, the sleep deprived ate more calories during after-dinner snacking than in any other meal during the day.”

As a whole, those who slept less ate 6 percent more calories. But after they got more sleep they switched to healthier foods with fewer carbs and less fat.

What’s more, lack of sleep changes your internal clock, which may explain the change in eating patterns. If you’re awakened early, the internal clock in your brain doesn’t realize that.

Wright says waking up the subjects early — during their biological nighttime — probably explains why they ate smaller breakfasts and more food after dinner. Their internal clocks were delayed at the end of the day.

There were differences between men and women. In general, men ate more, but both sexes gained weight.

Wright is quick to advise people not to use more sleep as a way to lose significant weight.

“If you’re overweight, just sleeping more may not help you lose weight,” he says. “You need to watch what you eat, exercise and sleep at least seven to nine hours a night regularly.”

Bob Eckel, a professor of medicine at the University of ŷ���ڱ���Ƶ School of Medicine who sees patients and served as one of the study’s researchers, agrees.

“Good sleep is just one part of good weight management,” Eckel says.

Eckel adds he’s always told his patients about the importance of sleep, but after the study, he’ll be telling them more often about sleep than he did before.

As for future sleep studies, Wright is working to learn more about the implications of when people are eating, not just what they’re eating.

And Eckel wants to know if we can make up for lost sleep. So, for example, if you’re slaving away for your boss during the week, can you make up for it over the weekend?

These are questions that, for now at least, we’ll just have to sleep on.

Illustration courtesy of Zack Blanton