title: Insomnia symptoms as a cause of type 2 diabetes Incidence: a 20 year cohort study.
abstract: Insomnia symptoms are associated with type 2 diabetes incidence but are also associated with a range of potential time-varying covariates which may confound and/or mediate associations. We aimed to assess whether cumulative exposure to insomnia symptoms has a causal effect on type 2 diabetes incidence.
  
  
  
  A prospective cohort study in the West of Scotland, following respondents for 20 years from age 36. 996 respondents were free of diabetes at baseline and had valid data from up to four follow-up visits. Type 2 diabetes was assessed at the final visit by self-report, taking diabetic medication, or blood-test (HbA1c≥ 6.5% or 48 mmol/mol). Effects of cumulative insomnia exposure on type 2 diabetes incidence were estimated with traditional regression and marginal structural models, adjusting for time-dependent confounding (smoking, diet, physical inactivity, obesity, heavy drinking, psychiatric distress) as well as for gender and baseline occupational class.
  
  
  
  Traditional regression yielded an odds ratio (OR) of 1.34 (95% CI: 1.06-1.70) for type 2 diabetes incidence for each additional survey wave in which insomnia was reported. Marginal structural models adjusted for prior covariates (assuming concurrently measured covariates were potential mediators), reduced this OR to 1.20 (95% CI: 0.98-1.46), and when concurrent covariates were also included (viewing them as potential confounders) this dropped further to 1.08 (95% CI: 0.85-1.37).
  
  
  
  The association between cumulative experience of insomnia and type 2 diabetes incidence appeared confounded. Evidence for a residual causal effect depended on assumptions as to whether concurrently measured covariates were confounders or mediators.
authors.affiliation: MRC/CSO Social & Public Health Sciences Unit, University of Glasgow, 200 Renfield Street, Glasgow, G2 3QB, UK. michael.green@glasgow.ac.uk.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Green
authors.affiliation: Nuffield Department of Clinical Neurosciences, Sleep & Circadian Neuroscience Institute, University of Oxford, Oxford, OX3 9DU, UK.
authors.forename: Colin A
authors.initials: CA
authors.lastname: Espie
authors.affiliation: MRC/CSO Social & Public Health Sciences Unit, University of Glasgow, 200 Renfield Street, Glasgow, G2 3QB, UK.
authors.forename: Frank
authors.initials: F
authors.lastname: Popham
authors.affiliation: Faculty of Health Sciences and Sport, University of Stirling, Stirling, FK9 4LA, UK.
authors.forename: Tony
authors.initials: T
authors.lastname: Robertson
authors.affiliation: Institute for Social and Economic Research, University of Essex, Colchester, CO4 3SQ, UK.|Institute of Health and Wellbeing, University of Glasgow, 1 Lilybank Gardens, Glasgow, G12 8RZ, UK.
authors.forename: Michaela
authors.initials: M
authors.lastname: Benzeval
mesh_terms: D000328:Adult; D000368:Aged; D015331:Cohort Studies; D003924:Diabetes Mellitus, Type 2 / Q000453:epidemiology* / Q000209:etiology*; D005260:Female; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D009765:Obesity / Q000150:complications / Q000453:epidemiology; D011446:Prospective Studies; D012307:Risk Factors; D012606:Scotland; D007319:Sleep Initiation and Maintenance Disorders / Q000150:complications* / Q000453:epidemiology*
keywords: Causal Effects; Confounding; Insomnia; Longitudinal; Marginal Structural Models; Sleep; Type 2 Diabetes