Longitudinal associations of sleep duration, morning and evening cortisol, and body mass index during childhood

OBJECTIVE. The primary objective of this paper was to examine associations between sleep duration, body mass index (BMI), and cortisol levels across childhood. METHODS. Participants included 361 children adopted domestically in the United States. Random-intercept cross-lagged panel models tested for between- and bi-directional within-person associations of sleep duration, BMI, and morning and evening cortisol from 4.5 to 9 years of age. RESULTS. Sleep duration and BMI were stable during childhood, inversely associated at the between-person level, and unrelated to morning or evening cortisol. BMI at 6 years predicted longer sleep duration and lower evening cortisol at 7 years, and lower morning cortisol at 7 years predicted higher BMI at 9 years, within individuals. CONCLUSIONS. The association between sleep and BMI is more likely a stable between-person phenomenon, rather than a unidirectional association that develops within-individuals over time.