Insomnia affects future development of depression in workers: a 6-year cohort study

The purpose of this study was to investigate the relationship between insomnia and future risk of developing depression. This was a 6-year cohort survey from 2011 to 2017. A questionnaire was conducted with male workers in a manufacturing industry. The questions included the Center for Epidemiologic Studies Depression Scale (CES-D) for evaluating depression and the Athens Insomnia Scale (AIS) for rating insomnia. Data from 1,332 daytime workers aged less than 60 years who had no depressive symptoms at baseline were analyzed. The risk of developing depression in the future was associated with insomnia at baseline, after adjusting for age (hazard ratio [HR] 1.64). Even after adjusting for the covariances of job type, living with family, sleeping time, and undergoing treatment or taking medication, insomnia was associated with the onset of future depression (HR 1.58). In addition, the HR increased as the total AIS score increased: total AIS score 1–3 points (HR 1.99; 95% CI 1.23–3.22), 4–5 points (HR 3.58; 95% CI 2.18–5.89), and 6 points and above (HR 4.24; 95% CI 2.49–7.21). The risk of developing depression in the future increased in correlation with greater severity of insomnia at baseline, suggesting that even slight insomnia can be a risk of future developing depression. It may be important to measure the level of insomnia using an indicator such as AIS, and to improve sleep quality in workers to prevent depression.