title: A twin study of sleep duration and body mass index.
abstract: OBJECTIVE
  To determine the relative importance of genetic and environmental contributions to the association between sleep duration and body mass index (BMI).
  
  
  METHODS
  Twins from the University of Washington Twin Registry, a community-based sample of U.S. twins, provided self-reported height and weight for BMI calculation and habitual sleep duration. A generalized estimating equation model evaluated the overall and within twin pair effects of sleep duration on BMI with and without stratification by twin zygosity. A structural equation model was used to assess genetic and non-genetic contributions to BMI and sleep duration.
  
  
  RESULTS
  The study sample included 1,224 twins comprised of 423 monozygotic, 143 dizygotic, and 46 indeterminate pairs. The mean age was 36.9 years; 69% were female. A multivariate adjusted analysis of all twins revealed an elevated mean BMI (26.0 kg/m2) in short sleeping twins (< 7 h/night) compared to twins sleeping 7-8.9 h/night (BMI 24.8 kg/m2; p < 0.01). The within-twin pair analysis revealed similar results, with the short sleeping twins having a mean BMI of 25.8 kg/ m2 compared to 24.9 kg/m2 for the 7-8.9 h/night sleep duration group (p = 0.02). When restricted to monozygotic twins, the within-twin pair analysis continued to reveal an elevated BMI in the short sleeping twins (25.7 kg/m2) compared to the 7-8.9 h/night reference group (24.7 kg/m2; p = 0.02). No differences in mean BMI were observed between the 7-8.9 h/night reference group twins and longer sleeping twins (> or = 9 h/night) in the analysis of all twins, the overall within-twin pair analysis, or the within-twin pair analysis stratified by zygosity. The heritability of sleep duration was 0.31 (p = 0.08) and BMI 0.76 (p < 0.01). Bivariate genetic analysis revealed little evidence of shared genetics between sleep duration and BMI (p = 0.28).
  
  
  CONCLUSIONS
  Short sleep was associated with elevated BMI following careful adjustment for genetics and shared environment. These findings point toward an environmental cause of the relationship between sleep duration and BMI.
authors.affiliation: Department of Neurology and University of Washington Medicine Sleep Institute, Seattle, WA 98104-2499, USA. nwatson@uw.edu
authors.forename: Nathaniel F
authors.initials: NF
authors.lastname: Watson
authors.forename: Dedra
authors.initials: D
authors.lastname: Buchwald
authors.forename: Michael V
authors.initials: MV
authors.lastname: Vitiello
authors.forename: Carolyn
authors.initials: C
authors.lastname: Noonan
authors.forename: Jack
authors.initials: J
authors.lastname: Goldberg
mesh_terms: D000328:Adult; D015992:Body Mass Index; D005260:Female; D006801:Humans; D008297:Male; D015999:Multivariate Analysis; D009765:Obesity / Q000235:genetics / Q000503:physiopathology; D012890:Sleep / Q000235:genetics* / Q000502:physiology*; D012893:Sleep Wake Disorders / Q000235:genetics / Q000503:physiopathology; D013997:Time Factors; D014481:United States