title: Sleep duration or bedtime? Exploring the association between sleep timing behaviour, diet and BMI in children and adolescents.
abstract: OBJECTIVE
  To determine whether sleep timing behaviour is associated with energy intake and diet quality in children and adolescents.
  
  
  METHODS
  Cross-sectional analysis of nationally representative survey data.
  
  
  METHODS
  A total of 2200 participants of the 2007 Australian National Children's Nutrition and Physical Activity Survey aged 9-16 years with 2 days of food intake data, 4 days of use of time data and complete anthropometry. Participants were grouped into one of four sleep-wake behaviour categories: early bed-early rise (EE); early bed-late rise (EL); late bed-early rise (LE) and late bed-late rise (LL). The four categories were compared for body mass index (BMI) z-score, energy intake and diet quality assessed using the Dietary Guideline Index for Children and Adolescents. Analyses were adjusted for survey design, sociodemographic characteristics, sleep duration and physical activity level (PAL).
  
  
  RESULTS
  In adjusted multivariate regression models with sleep timing behaviour group as the independent variable, the 'LL' category compared with the 'EE' category had a higher BMI z-score (β=0.20, 95% confidence interval (CI) 0.06 to 0.34, P=0.007), and lower diet quality (β=-4.0, 95% CI -5.7 to -2.3, P<0.001). Children and adolescents who went to bed late also had a higher intake of extra foods (that is, energy-dense, nutrient-poor foods) while those whom went to bed early consumed more fruit and vegetables. Energy intake was associated with sleep duration (β=-4.5 kJ, 95% CI -6.7 to -2.4, P<0.001), but not sleep timing behaviour.
  
  
  CONCLUSIONS
  Late bedtimes and late wake up times are associated with poorer diet quality, independent of sleep duration, PAL and child and sociodemographic characteristics.
authors.affiliation: School of Pharmacy and Medical Sciences, University of South Australia, Adelaide, South Australia. rebecca.golley@unisa.edu.au
authors.forename: R K
authors.initials: RK
authors.lastname: Golley
authors.forename: C A
authors.initials: CA
authors.lastname: Maher
authors.forename: L
authors.initials: L
authors.lastname: Matricciani
authors.forename: T S
authors.initials: TS
authors.lastname: Olds
mesh_terms: D000293:Adolescent; D001315:Australia / Q000453:epidemiology; D015992:Body Mass Index; D002648:Child; D002664:Child Nutritional Physiological Phenomena; D002940:Circadian Rhythm / Q000502:physiology*; D003430:Cross-Sectional Studies; D004435:Eating; D005247:Feeding Behavior; D005260:Female; D006801:Humans; D008297:Male; D009043:Motor Activity; D009749:Nutrition Surveys; D009752:Nutritional Status; D009765:Obesity / Q000150:complications* / Q000453:epidemiology; D012890:Sleep; D012931:Social Environment