Effect of the interaction between employment level and psychosocial work environment on insomnia in male Japanese public service workers.

BACKGROUND: Little is known about the combined risk of socioeconomic status and psychosocial environment for insomnia.
PURPOSE: 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.
METHOD: 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.
CONCLUSION: 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.