School Start Time and the Healthy Weight of Adolescents.

Purpose: Studies have found that an early school start time is detrimental to the sleep, health, and well-being of youth, but its association with body weight remains unclear. We examined this association in Canadian adolescents.

Methods: We collected information on start times from 362 schools that participated in the 2013/2014 Canadian Health Behaviour in School-aged Children Study (n = 29,635 students; ages 10-18). We estimated body mass indices (BMIs) and BMI z-scores, and identified overweight and obesity using international growth references. Multilevel regression models tested the associations between school start times and our outcomes, adjusted for grade, family affluence, school rurality, latitude, and province.

Results: The average BMI was 21.2 (standard deviation 4.9) and BMI z-score was .48 (standard deviation 1.23). Every 10-minute delay in school start time corresponded with a .02 (95% confidence interval .00, .04) smaller BMI z-score. This association translated to BMIs in the 70th and 64th percentiles when comparing students from schools that started at 8:00 a.m. and 9:30 a.m., respectively. School start time was not significantly related to overweight or obesity.

Conclusions: Later school start time was linked to lower BMI in Canadian adolescents. Delaying school start time may be an additional strategy to support the healthy weight of adolescents. Future intervention and impact studies are recommended to confirm these findings.