Friday, May 3, 2013

Sleep duration and adolescent obesity

Another story about sleep duration and obesity. This study looked at 1390 kids from age 14 to 18 years old. They asked them to report their height, weight, number of hours they sleep per night, physical activity level, and amount of time spent in front of a screen (TV / video games / computer) every 6 months. The average amount of sleep per day was about 8 hours and this decreased to 7.5 hours by the time the kids reached age 18. When the researchers crunched the numbers, they saw that longer sleep duration was associated with a lower body mass index (BMI), a ratio of height to weight. The higher the BMI, the more overweight the kid. The association between BMI and sleep duration was stronger as the BMI increased - for each additional hour of sleep past 8 hours, the BMI was lower than the average. This means that more sleep seemed to help the most overweight kids.

From these results, the authors suggest that kids who sleep at least 10 hours per day could reduce their BMI. Interestingly, the association between BMI and sleep duration persisted even when accounting for physical activity and screen time. So even if overweight kids watch TV as much as normal weight kids, increased sleep duration may reduce obesity.

One important limitation in this study is that they did not monitor calorie consumption. So, like I've mentioned in other posts, longer sleep may be associated with less weight because of decreased calorie consumption, not from longer sleep itself.

The authors point out that telling kids to sleep more does not work. But delaying school start time in the morning may allow kids to sleep more. Also, getting all electronic devices out of the bedroom at night may increase sleep duration.

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