Stay Up Late, Add Some Weight, Study Says

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d57fa3c5-906b-4922-b524-93a4bcdcaf6dWhether you're an active teenager or a slow-moving young adult, the later you go to sleep the greater the chances that you'll gain weight as compared to those who hit the sack earlier. This is according to a study from the University of California, Berkeley, which may have found a correlation between sleep and body mass index (BMI).

The Berkeley team analyzed data on more than 3,300 teens and young adults recorded over 15 years, and found that over a five year period, an individual s body BMI increased 2.1 points for every hour of sleep consistently lost.

A healthy adult BMI range is expected to be between 18.5 and 24.9. A BMI of 25 to 29.9 indicates being overweight and 30 or above puts you into the obese category, according to the National Institutes of Health.

"Conceivably, if you're going to bed an hour later, over time you could be shifting BMI categories from normal to overweight," said Asarnow, a UC Berkeley doctoral candidate. "So even a two-point increase could be clinically significant," she added, speaking with CBS News.

The researchers were surprised to find that the correlation between bedtime and BMI was not significantly impacted either way, by exercise, computer and TV screen time, or the total hours of sleep time.

The reason late bedtimes and BMI are correlated is unknown, but researchers believed that it has to do with a combination of both metabolic and behavioral patterns. Likewise, going to bed late can shift the body s biological clock, which can lead to eating habits to change. "If you're staying up late you're more likely to be eating junk food late at night," Asarnow said. "People who stay up late are also less likely to eat breakfast and breakfast skipping is associated with weight gain."

However, while presenting their findings, the New York Times points out the "scientists acknowledge that their study had limitations. Their sleep data depended on self-reports, and they did not have complete diet information. Also, they had no data on waist circumference, which, unlike B.M.I., can help distinguish between lean muscle and abdominal fat."

According to the National Sleep Foundation, teenagers over the age of 14 are recommended to get eight to 10 hours of sleep each night, while young adults over the age of 20 need seven to nine hours. But the Center for Disease Control and Prevention reported that only 31 percent of high school students said they are getting at least eight hours of sleep each night, while practically 30 percent of adults said their sleeping less than six hours each night.