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Study: People Who Eat After 8 p.m. Have Higher BMIs

June 14, 2011 (Minneapolis) -- Checking a clock may be as helpful as counting calories when it comes to controlling body weight, a new study suggests.

The study shows that people who snack after 8 p.m. have higher body mass indexes (BMIs) than people who don’t nosh at night, even though they don’t eat significantly more total daily calories.

Previous studies in animals have found that even when calories are held steady, the timing of meals and sleep and exposure to light can impact metabolism and BMI.

The new study is one of the first to explore those relationships in humans.

Tracking Mealtimes, Sleep, and BMI

Researchers at Northwestern University in Chicago recruited 52 young and middle-aged adults.

Study participants wore sensors around their wrist that recorded movement and sleep times throughout the day for seven days.

They kept food diaries to track what, when, and how much they were eating.

Night owls tended to be late sleepers, with a midpoint of sleep that was after 5:30 a.m.

Late sleepers typically logged less sleep than normal sleepers. They also started their days later, a pattern that pushed back mealtimes throughout the day.

Additionally, they had higher BMIs than normal sleepers, ate more calories after 8 p.m., and ate fewer fruits and vegetables.

Researchers took into account factors that are known to increase the risk of weight gain, like age, sleep duration, and sleep timing.

“After we adjusted for all of those things, the one major thing that remained positive, that remained correlated, was eating after 8 p.m.,” says study researcher Phyllis Zee, MD, associate director of the Center for Sleep and Circadian Biology and a professor at Northwestern University’s Institute for Neuroscience in Chicago.

“Although those late sleepers were eating more calories, the number of calories between the normal sleepers and the late sleepers was not significantly different,” Zee tells WebMD.

The study was presented at the 2011 annual meeting of the American Academy of Sleep Medicine and the Sleep Research Society in Minneapolis.

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