title: Effect of the interaction between employment level and psychosocial work environment on insomnia in male Japanese public service workers.
abstract: BACKGROUND
  Little is known about the combined risk of socioeconomic status and psychosocial environment for insomnia.
  
  
  OBJECTIVE
  The purpose of this study was to investigate the combined risk of employment level and psychosocial work environment for insomnia in a large Japanese male working population.
  
  
  METHODS
  We investigated 5,951 male employees aged 34-59 years from two local governments in Japan. Data were obtained from a questionnaire distributed in advance of their annual health check-up. Employment level was categorized as higher-level non-manual workers, lower-level non-manual workers, and manual workers. Psychosocial work environment was assessed using the Demand-Control Model and the Effort-Reward Imbalance Model (ERI). Insomnia was assessed using the Athens Insomnia Scale.
  
  
  RESULTS
  The results showed that 1,382 (23.2 %) participants suffered from insomnia. Lower employment level was significantly associated with a higher risk of insomnia. Job strain (ratio of job demand to job control), E/R (ratio of job effort to job reward), and OC (over commitment) were also significantly associated with insomnia. The relative excess risk due to the interaction between employment level and psychosocial environment (job strain, E/R, and OC) was 0.09 (95 % CI -0.57 to 0.76), 2.61 (0.74 to 4.48), and 3.14 (0.82 to 5.45), respectively; synergy index = 1.07 (0.66 to 1.74), 1.99 (1.37 to 2.90), and 2.25 (1.46 to 3.46), respectively.
  
  
  CONCLUSIONS
  We found supra-additive interactions between employment level and psychosocial environment, assessed by the ERI (E/R and OC). Our findings suggested that lower level workers are more vulnerable to an adverse psychosocial environment than those at a higher level.
authors.affiliation: Department of Health Science, Asahikawa Medical University, Midorigaoka E2-1-1-1, Asahikawa, Hokkaido, 078-8510, Japan, e-yoshi@asahikawa-med.ac.jp.
authors.forename: Eiji
authors.initials: E
authors.lastname: Yoshioka
authors.forename: Yasuaki
authors.initials: Y
authors.lastname: Saijo
authors.forename: Toshiko
authors.initials: T
authors.lastname: Kita
authors.forename: Hiroki
authors.initials: H
authors.lastname: Satoh
authors.forename: Mariko
authors.initials: M
authors.lastname: Kawaharada
authors.forename: Reiko
authors.initials: R
authors.lastname: Kishi
mesh_terms: D000328:Adult; D044466:Asian People / Q000523:psychology* / Q000706:statistics & numerical data; D004651:Employment / Q000523:psychology*; D006801:Humans; D007564:Japan / Q000453:epidemiology; D008297:Male; D008875:Middle Aged; D009790:Occupations; D015995:Prevalence; D017150:Public Sector; D012307:Risk Factors; D007319:Sleep Initiation and Maintenance Disorders / Q000453:epidemiology / Q000523:psychology*; D012923:Social Class; D012931:Social Environment; D013315:Stress, Psychological / Q000453:epidemiology / Q000523:psychology*; D011795:Surveys and Questionnaires; D017132:Workplace / Q000523:psychology; D055815:Young Adult