Evidence for a Possible Link between Bedtime and Change in Body Mass Index

Objectives: The aim of the current study was to examine the longitudinal relationship between bedtimes and body mass index (BMI) from adolescence to adulthood in a nationally representative sample. Design: Three waves of data from the National Longitudinal Study of Adolescent Health were used to assess the bedtimes and BMI of 3,342 adolescents between 1994 and 2009. Hypotheses were tested with hierarchical linear models using a two-level, random intercept and slopes model. Results: Later average bedtime during the workweek, in hours, from adolescence to adulthood was associated with an increase in BMI over time (b = 0.035 kg/m(2) per min later bedtime per 6 years; standard error = 0.016; t = 2.12, degrees of freedom = 3,238, P < 0.05). These results remained significant after controlling for demographic characteristics and baseline BMI. Although sleep duration, screen time, and exercise frequency did not attenuate the relationship between workday bedtime and BMI over time, fast-food consumption was recognized as a significant partial mediator of the relationship between bedtimes and BMI longitudinally. Conclusions: The results highlight bedtimes as a potential target for weight management during adolescence and during the transition to adulthood.