title: Joint associations of sleep duration and insomnia symptoms with subsequent sickness absence: the Helsinki Health Study.
abstract: OBJECTIVE
  We aimed to examine the joint associations of sleep duration and insomnia symptoms with subsequent sickness absence of various lengths while considering several covariates.
  
  
  METHODS
  Baseline surveys among 40-60-year-old employees of the City of Helsinki, Finland, (N = 6535) were prospectively linked with employer's personnel register data comprising short self-certified (1-3 days), medically-certified intermediate (4-14 days) and long (15 days or more) sickness absence spells. Average follow-up time was 4.1 years. Sleep duration, insomnia symptoms, sociodemographics, working conditions, health behaviours and health were self-reported in the surveys. Poisson regression analysis was used.
  
  
  RESULTS
  Insomnia symptoms were associated with sickness absence at all levels of sleep duration. Adjusting for gender and age, U-shaped associations regarding sleep hours were found. Thus, those reporting short or long sleep and reporting insomnia symptoms had a higher risk for medically-certified intermediate and long sickness absence as compared to those reporting 7 hours of sleep without insomnia symptoms. Also, those reporting 6, 7, and 8 hours of sleep had a higher risk for such sickness absence, if they reported insomnia. Weak associations were also found for self-certified sickness absence, and for those reporting short and long sleep without insomnia. Adjustments attenuated the associations, but they mainly remained.
  
  
  CONCLUSIONS
  These results suggest primacy of the effects of insomnia symptoms over sleep duration on sickness absence. Although insomnia dominated the joint association, U-shaped associations suggest that both sleep duration and insomnia symptoms need to be considered to promote work ability.
authors.affiliation: Hjelt Institute, Department of Public Health, University of Helsinki, Finland. tea.lallukka@ttl.fi
authors.forename: Tea
authors.initials: T
authors.lastname: Lallukka
authors.forename: Peija
authors.initials: P
authors.lastname: Haaramo
authors.forename: Ossi
authors.initials: O
authors.lastname: Rahkonen
authors.forename: Børge
authors.initials: B
authors.lastname: Sivertsen
mesh_terms: D000328:Adult; D005260:Female; D005387:Finland; D005500:Follow-Up Studies; D006306:Health Surveys; D006801:Humans; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D018582:Sick Leave / Q000706:statistics & numerical data*; D012890:Sleep / Q000502:physiology*; D007319:Sleep Initiation and Maintenance Disorders / Q000150:complications*; D013997:Time Factors
keywords: employees; follow-up; insomnia; register-based; sleep duration; work disability