Abstract
OBJECTIVE: To test the independent main and moderating effects of sleep timing on body mass index (BMI) in low-income preschool-age children (M = 4.11 years, SD = 0.54). METHODS: Parents reported demographics and children's sleep concurrently, and a subset of children was followed longitudinally. Child height and weight were measured and BMI z score (BMIz) calculated. Regression analysis evaluated main effects of sleep timing (bedtime, weekday-to-weekend schedule shifting, napping) on concurrent BMIz and future rate of change, and their moderating effects on the sleep duration-BMIz association. RESULTS: Of 366 children (longitudinal subsample = 273), 50% were boys, 57% white, and 37% overweight or obese. Nocturnal sleep duration predicted concurrent BMIz, but not rate of change in BMIz over time. Bedtime was a moderator; the sleep duration-BMIz association was present only among children with bedtimes after 9 pm (β = -0.44; 95% confidence interval -0.69, -0.18). Schedule shifting was a moderator; the association between greater nocturnal sleep duration and lesser rate of future BMIz increase was present only among children with the most consistent sleep schedules (<45-minute delay in weekend bedtime: β = -0.12; 95% confidence interval -0.23, -0.01). Daytime napping did not moderate the nocturnal sleep duration-BMIz association. Covariates (sleep-disordered breathing, soda consumption, home chaos) did not explain these associations. CONCLUSIONS: Among low-income preschoolers, sleep timing moderated the nocturnal sleep duration-BMIz association. Understanding how sleep timing and sleep duration relate to childhood obesity is important for prevention efforts.