title: Sleep timing moderates the concurrent sleep duration-body mass index association in low-income preschool-age children.
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.
authors.affiliation: School of Public Health, University of Michigan, Ann Arbor, Mich; Center for Human Growth and Development, University of Michigan, Ann Arbor, Mich. Electronic address: alimill@umich.edu.
authors.forename: Alison L
authors.initials: AL
authors.lastname: Miller
authors.affiliation: Center for Human Growth and Development, University of Michigan, Ann Arbor, Mich.
authors.forename: Niko
authors.initials: N
authors.lastname: Kaciroti
authors.affiliation: Department of Integrative Physiology, University of Colorado, Boulder, Colo.
authors.forename: Monique K
authors.initials: MK
authors.lastname: Lebourgeois
authors.affiliation: School of Public Health, University of Michigan, Ann Arbor, Mich.
authors.forename: Yu Pu
authors.initials: YP
authors.lastname: Chen
authors.affiliation: Center for Human Growth and Development, University of Michigan, Ann Arbor, Mich.
authors.forename: Julie
authors.initials: J
authors.lastname: Sturza
authors.affiliation: School of Public Health, University of Michigan, Ann Arbor, Mich; Center for Human Growth and Development, University of Michigan, Ann Arbor, Mich; Department of Pediatrics, University of Michigan, Ann Arbor, Mich.
authors.forename: Julie C
authors.initials: JC
authors.lastname: Lumeng
mesh_terms: D015992:Body Mass Index; D002675:Child, Preschool; D005260:Female; D006801:Humans; D008297:Male; D009765:Obesity / Q000453:epidemiology / Q000208:ethnology; D050177:Overweight / Q000453:epidemiology / Q000208:ethnology; D011203:Poverty; D012044:Regression Analysis; D012890:Sleep / Q000502:physiology*; D012959:Socioeconomic Factors; D013997:Time Factors
keywords: low-income preschool-age children; obesity; sleep duration; sleep timing