Background: Depression influences a worker's productivity and health substantially. Recently, the Japanese society and government reported that working overtime is one of the primary causes of depression and suicide in workers. However, only a few studies have investigated the relation between overtime hours and mental health status, and conclusions vary. In addition, prior findings are inconsistent in terms of the relation between depression and lifestyle factors, including alcohol intake and smoking. Additional studies are required to clarify the relation between possible risk factors and depression in Japanese workers. Methods: We performed a case-control and a cohort study. Subjects were office workers in four Japanese companies. Diagnosis of depression was made by two psychiatrists who conducted independent clinical interviews using DSM-IV-TR criteria. Results: There was no significant association between working overtime and the onset of depression. The frequency of alcohol intake was significantly related to the onset of depression. We also found a significant relation between younger age and depression onset. Body mass index and physical illness, including diabetes mellitus, had no significant association with depression onset. Limitations: Data were self-reported and the number of included female workers was small. Conclusions: Reducing working hours alone is unlikely to be effective in preventing workers' depression. Additional countermeasures are needed, including a reduction in alcohol intake and work stress. Considerations for younger workers are also needed.
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