The Effect of Sleep Duration on Body Weight in Adolescents: Evidence from a Natural Experiment

Despite a large number of observational studies consistently reporting the association between shorter sleep duration and higher body weight, causality has yet to be established at a population level. This study aims to estimate the population-level causal effect of sleep duration on adolescent body weight, using an instrumental variable approach that exploits a unique natural experiment in the context of South Korea’s highly competitive secondary education.

In March 2011, amid growing concerns over the negative consequences of late-night tuition at private tutoring institutes (hagwon), authorities in 3 of the 16 administrative regions in South Korea decreed shortening the operating hours of hagwon by two hours (adjusting the closing hours from midnight to 10 p.m.). This 2-hour shift in hagwon curfew caused a substantial and plausibly exogenous variation in the sleep duration of the “marginal student.” General high school (GHS) students in the 10^th and 11^th grades are most likely to be the marginal students. Many of them use hagwons to prepare for the College Scholastic Ability Test and tend to reduce their sleep hours to have more time for taking extra courses at hagwons. However, these students still have one year left before the entrance exam, which is taken toward the end of 12^th grade; hence, GHS 10^th-and 11^th-graders are not desperate enough to sacrifice sleep hours for more study time at every opportunity. Therefore, GHS 10^th- and 11^th-graders are likely to get more sleep in response to the policy restricting hagwon operation hours, providing the source of identification necessary for the IV estimation in this study.

The current study used data from four annual surveys of the 2009−2012 Korea Youth Risk Behavior Web-based Survey, a repeated cross-sectional, school-based, online survey of a large, nationally representative sample of middle-school and high school students (aged 13 through 18 years) in South Korea. The instrumental variable estimation results on the marginal student sample of GHS 10^th- and 11^th-graders (N=52,585) showed that a 1-hour increase in sleep duration led to a 0.56 kg/m² reduction in body mass index, or a 4.3 percentage-point decrease in overweight/obesity. Short sleep duration among adolescents may be an important contributor to increased body weight at the population level.