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By Benjamin Plackett

SUNDAY, June 10, 2012 ( — As any college student or shift worker will tell you, staying up all night or even just skimping on sleep can lead a person to seek out satisfying, calorie-packed foods.

An emerging body of research suggests that sleep-related hunger and food cravings, which may contribute to weight gain, are fuelled in part by certain gut hormones involved in appetite. But our brain, and not just our belly, may play a role as well.

According to two small studies presented today at a meeting of sleep researchers in Boston, sleep deprivation appears to increase activity in areas of the brain that seek out pleasure—including that provided by junk food. To make matters worse, sleepiness also may dampen activity in other brain regions that usually serve as a brake on this type of craving.

In one of the studies, researchers at Columbia University used functional magnetic resonance imaging (fMRI), which tracks blood flow in the brain, to compare brain activity in 25 volunteers following a normal night's sleep (about eight hours) and a night in which they were limited to just four hours.

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In each case, the researchers performed the scans while showing the volunteers images of unhealthy foods interspersed with healthy foods, such as fruits, vegetables, and oatmeal. Brain networks associated with craving and reward were more active when the participants were sleep-deprived than when they were well-rested—especially when the participants viewed the images of unhealthy foods.

"The pleasure-seeking parts of the brain were stimulated after an individual was sleep-deprived," says lead researcher Marie-Pierre St-Onge, Ph.D., a research associate at the university's New York Obesity Research Center. "People went for foods like pepperoni pizza, cheeseburgers, and cake."

St-Onge and other researchers working in this field suspect that tired people gravitate to high-calorie foods because their bodies and brains are seeking an extra energy boost to help them get through the day. "We hypothesize that the restricted-sleep brain reacts to food stimuli as though it [were] food deprived," St-Onge says.

Previous studies have established a link between sleep deprivation and obesity, although it remains unclear how sleep might affect weight gain (or vice versa). In an effort to unravel the relationship, researchers have begun exploring how insufficient sleep influences hormones and appetite. Several recent studies—including one led by St-Onge—have found that people who are sleep deprived tend to snack more and consume more calories.

Hunger and cravings may not be the only factors, however. A second study presented today suggests that so-called higher-order brain functions—those that help up us weigh pros and cons and make complex choices, including about what we eat—may be compromised by a lack of sleep.

The study was similar to the one conducted at Columbia. Researchers at the University of California, Berkeley asked 23 healthy young adults to rate their desire for various foods while undergoing fMRI. The participants expressed a stronger preference for unhealthy food when they'd been awake for 24 hours, compared to when they were well-rested.

But the brain scans added a new wrinkle: When the volunteers were sleep-deprived, their brains showed diminished activity in networks involved in decision-making, not simply increased activity in pleasure-seeking areas.

"We did see pleasure-seeking regions stimulated, but just not any more so than other regions," says Stephanie Greer, a researcher at the Berkeley Sleep and Neuroimaging Laboratory, who led the study.

This finding suggests that tired people are drawn to fatty and high-calorie foods partly because their ability to process information and make decisions is impaired. "Fewer subjects…took health and taste into account" when rating their food preferences after a sleepless night, Greer says.

Michelle Miller, Ph.D., a sleep researcher at the University of Warwick Medical School, in the U.K., says the disparity in the two sets of findings might be explained by the severity of the sleep deprivation. The participants in the Columbia study got four hours of sleep, whereas those in the Berkeley study got no sleep at all.

Pleasure-seeking and impaired decision-making both may play a role in food cravings, Miller says, but the latter might become more important as sleep deprivation worsens.

St-Onge and Greer presented their findings at the annual meeting of the American Academy of Sleep Medicine. Both studies will need to be confirmed in future research. They were on the small side—which isn't unusual for fMRI studies, Miller points out—and the patterns seen in fMRI studies don't always translate into real-world behavior.