Later bedtimes may lead to an increase in body mass index over time

A new study suggests that going to bed late during the workweek from adolescence to adulthood is associated with an increase in body mass index over time.

Results of hierarchal linear models involving a nationally representative sample of more than 3,000 participants show that going to bed during the workweek each additional hour later is associated with an increase in BMI of 2.1 kg/m2. Moreover, surprising to the researchers, the relationship between bedtime and BMI was not significantly changed or moderated by total sleep time, exercise frequency or screen time.

"The results are important because they highlight adolescent bedtimes, not just total sleep time, as a potential target for weight management concurrently and in the transition to adulthood," said first author Lauren Asarnow, a doctoral candidate at the University of California, Berkeley.

Study results are published in the October issue of the journal Sleep.

Along with Eleanor McGlinchey, PhD, and Allison G. Harvey, PhD, Asarnow analyzed three waves of data from the National Longitudinal Study of Adolescent Health (Add Health) to assess the bedtimes and BMI of 3,342 adolescents between 1994 and 2009. Sleep and circadian variables were determined via self-reported measures at all three waves. Investigators measured height and weight at each wave, from which BMI was calculated.

According to the authors, this is the first study to examine the longitudinal relationship between bedtimes and BMI in any age group in an observational study.

The American Academy of Sleep Medicine recommends that adolescents get a little more than nine hours of nightly sleep for optimal health and daytime alertness during the critical transition from childhood to adulthood.