Introduction Overtime work has been suspected as a risk factor for workers’ mental distress. A recent cross-sectional study showed that job satisfaction and overtime work could modify the association with mental health in a direct and combined manner (Nakata, 2017). Thus, we have examined the association between job satisfaction and overtime work with mental health using a longitudinal study to explore of causal relationships.

Methods Participants were 1,558 Japanese IT (information technology) workers. Each completed the General Health Questionnaire (GHQ, 28-item version) and Brief Job Stress Questionnaire (BJSQ, index for job strain and job supports) and indicated job satisfaction in 2011 and 2012. Actual amount of overtime work during 3 months in 2011 was recorded by the company. A total of 1408 respondents completed questionnaires and 758 participants were excluded from this analysis because of high GHQ scores (GHQ >5, indicating mental distress) in 2011. Finally, 650 participants were used for the evaluation. Risk of mental distress in 2012 by overtime work, job satisfaction, and both combined was estimated by univariable and multivariable logistic regression analysis.

Result Compared to participants with high job satisfaction, those with low satisfaction had increased risk of mental distress in the following year (crude odds ratio (cOR): 1.75), but this difference became insignificant after adjusting for potential confounding factors such as job strain and job supports. Furthermore, there was no significant difference in the risk of mental distress for participants with or without overtime work. However, in the combined analysis, compared to participants with overtime work and high job satisfaction, those with overtime work and low job satisfaction had an increased risk of high GHQ scores (adjusted OR: 2.04).

Conclusion Present longitudinal data suggests that the combination of low job satisfaction and overtime work induces an increased risk of mental distress.