When you look at a greasy cheeseburger, or a decadent dessert, your brain's reward centres becomes aroused. But it gets even more excited at the sight of unhealthy fare when you haven't had enough sleep.
This is what researchers from Columbia University in New York City found, after putting 25 men and women of healthy weights in a functional MRI scanner and showing them images of food.
But first, participants stayed in the lab for five nights, either getting a nightly four hours of sleep, or an uninterrupted slumber of up to nine hours per night. A month later, the same participants were tested, but with their amounts of sleep reversed.
Their brains were observed as they looked at pictures of healthy foods like fruits and vegetables, oatmeal and yogurt, as well as unhealthy foods, including bacon cheeseburgers, pepperoni and donuts.
"Under restricted sleep, brain activity response to unhealthy food was stronger," says the study's lead investigator, Marie-Pierre St-Onge. "The brain regions involved in reward and pleasure-seeking behaviours were activated in greater extent when they were sleep-deprived compared to when they had a full-night's sleep."
While the diet was controlled for most of the week, the day before the brain scans were taken, the participants were allowed to choose their own food. When sleep-deprived, they ate more, and more fat in particular.
"This could be a good starting point to explain the link between sleep deprivation and obesity," says Reut Gruber, director of the Attention Behaviour and Sleep Laboratory at the Douglas Mental Health University Institute in Montreal.
"It could explain why we tend to appreciate more food that is high in calories and sugar when we're sleep-deprived."
A strong proponent of the importance of sleep, Gruber chairs the pediatric sleep interest group Canadian Sleep Society. She has developed a school-based program in Montreal's Riverside School Board to help encourage healthy sleep habits.
She has also recently received grant money to integrate a sleep component into child-obesity programming.
"One aspect is that sleep deprivation alters metabolism," says Gruber, "but another factor that contributes is that there is an increased consumption of this type of foods."
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St-Onge's research shows how the cause might reside in the overtired brain.
"If you're trying to watch your weight, being sleep-deprived is not a good thing for you," she says. "You have hormonal and neuronal networks that set you up against your success."
She'll be presenting her study, "Sleep restriction increases the neuronal response to unhealthy food stimuli," at the SLEEP 2012 conference running from Saturday to Wednesday. The conference is the Associated Professional Sleep Societies' annual meeting in Boston.
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