title: Sleep duration and adiposity during adolescence.
abstract: OBJECTIVE
  The association between sleep and obesity has been described in different age groups. However, there are not sufficient data to clarify the inconsistent results reported in adolescents. Our objective was to study the associations between sleep duration and adiposity at 13 and at 17 years of age, with both cross-sectional and longitudinal approaches.
  
  
  METHODS
  We evaluated, as part of an urban population-based cohort (EPITeen), 1171 adolescents at both 13 and 17 years of age. Sleep duration was estimated by self-reported bedtimes and wake-up times. Age- and gender-specific BMI z scores were calculated based on Centers for Disease Control and Prevention references. Body fat percentage (BF%) was assessed by bioelectrical impedance. Regression coefficients (β) and respective 95% confidence intervals (CIs) were used to estimate the association between sleep and BMI z scores and BF%. Additionally, a cross-lagged analysis was performed to investigate the causal relations.
  
  
  RESULTS
  In the cross-sectional analysis, at 13 years, sleep duration was inversely associated with BMI z score only in boys (β = -0.155, 95% CI: -0.267 to -0.043); at 17 years, a positive association was found among girls but was only significant for BF% (β = 0.510, 95% CI: 0.061-0.958). In the longitudinal approach, sleep duration at age 13 was inversely associated with BMI z score (β = -0.123, 95% CI: -0.233 to -0.012) and BF% (β = -0.731, 95% CI: -1.380 to -0.081) at 17 years only in boys. These significant associations disappeared after adjustment for adiposity at 13 years. These results were corroborated by those from cross-lagged analysis.
  
  
  CONCLUSIONS
  Our results showed an effect of sleep duration in adiposity at younger ages of adolescence and suggested gender differences in this association.
authors.affiliation: Department of Clinical Epidemiology, Predictive Medicine and Public Health, and Cardiovascular Research & Development Unity, University of Porto Medical School, Porto, Portugal. jfaraujo@med.up.pt
authors.forename: Joana
authors.initials: J
authors.lastname: Araújo
authors.forename: Milton
authors.initials: M
authors.lastname: Severo
authors.forename: Elisabete
authors.initials: E
authors.lastname: Ramos
mesh_terms: D050154:Adiposity; D000293:Adolescent; D015992:Body Mass Index; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D012890:Sleep; D013997:Time Factors; D014504:Urban Health