Less sleep, larger BMI increases for teens

Adequate sleep might be an important key in maintaining a healthy weight for teens. Increasing the number of hours of sleep to 10 hours each night may reduce the prevalence of adolescent obesity, according to a new study published online Monday in Pediatrics.

The study observed over 1,000 Philadelphia-area high school students from their freshmen through senior high school years. Every six months, the students were asked to report their sleep patterns. Heights and weights were taken to calculate their adolescent body mass index (BMI) .

Fewer hours of sleep was associated with greater increases in BMI for participants between 14 and 18-years-old, found researchers from the Perelman School of Medicine at the University of Pennsylvania.

The results showed sleeping for 10 hours per day versus less than 8 hours per day could reduce the proportion of adolescents with a BMI larger or equal to 25 by 3 percent at age 14 and by 4 to 6 percent at age 18. Previous studies have shown that a connection exists between short sleep and obesity, but until now few have been able to rule out other variables such as time spent watching television and being physically active affected BMI.

We asked one of the study’s authors Jonathan Mitchell, PhD, a postdoctoral fellow in the Center for Clinical Epidemiology and Biostatistics at Penn Medicine, to tell us more about the study.

Why does sleep duration affect BMI?

Several theories  have been proposed on this.  It has been suggested that those with short sleep duration engage in less daytime physical activity due to tiredness. Another thought is that those with short sleep consume more calories per day.  This could simply be a consequence of more time spent awake and therefore more eating opportunities.  Alternatively, this could be driven by changes in hormones that regulate energy homeostasis.

How does this study add to what we know about short sleep and obesity in adolescents?

Our study is one of the few that has included repeated measures of sleep duration and BMI during adolescence. Most studies have used a cross-sectional design, meaning that a single measure of sleep duration and BMI was obtained at one point in time. This is important because cross-sectional studies cannot determine the temporal sequence between sleep duration and obesity.  The longitudinal design in our study allowed for us to determine that sleep duration was associated with higher BMI growth trajectories from age 14 to 18.

Can you talk more about how each additional hour of sleep was associated with a reduction in BMI?

In our analysis, we observed an association between shorter sleep duration and increases in BMI at all percentiles.  However, the association was stronger at the upper percentiles compared to the lower percentiles of the BMI distribution. This means that increasing sleep duration among all adolescents could be especially beneficial for decreasing the prevalence of adolescent obesity.

Interestingly, the association between short sleep duration and higher BMI remained after adjusting for physical activity levels in our study.  This indicates that reduced physical activity does not fully explain the association we observed between short sleep duration and higher BMI.


What can parents do to help their teens get more sleep?

Educating adolescents on the benefits of sufficient sleep will not have a great impact on increasing their sleep duration.  Therefore, parents will likely have a role to play, particularly with respect to creating households that practice good sleep hygiene.  For example, parents could help adolescents to establish and keep routine bedtimes and wake-up times; and they could remove electronic devices, such as televisions and tablet computers, from bedrooms. However, parents can only do so much.  The early start of the high school day requires adolescents to wake earlier. Delaying the start of the high school day could help to increase adolescent sleep duration.

Is sleep duration more important than screen time and physical activity in affecting BMI?

Researchers are currently looking at the aspects associated with short sleep duration and higher BMI, such as screen time, physical activity and diet. Therefore, sleep duration is not more or less important than other obesity risk factors; rather they appear to be inter-related. More research is needed to better understand the complex relationship between daytime behavior and sleep.

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