Long Working Hours and Risk of Ischemic Heart Disease Among Japanese Workers: The Jichi Medical School Cohort Study

OBJECTIVES: Karoshi, or "death from overwork," has been recognized for decades; however, epidemiological findings regarding long working hours (LWH) and ischemic heart disease (IHD) remain inconsistent. This study aimed to provide new evidence on the association between LWH and IHD, while accounting for job strain among Japanese workers, both male and female. METHODS: This study utilized data from 6670 workers participating in the Jichi Medical School Cohort Study. Baseline working hours were categorized as <5.0, 5.0 to 6.9, 7.0 to 8.9 (reference), 9.0 to 10.9, and ≥11.0 hr/day. Fatal and non-fatal incident IHD cases were determined during follow-up using International Classification of Diseases, 10th revision codes. Multivariable Cox proportional hazards models were used to examine associations, adjusting for socio-demographic factors, lifestyle behaviors, cardiometabolic characteristics, and job strain. RESULTS: During a mean follow-up of 12 years, 58 incident IHD cases (42 male and 16 female) were documented. Long working hours (≥11.0 hr/day) were significantly associated with an increased risk of IHD in the total sample (hazard ratio, 2.92; 95% confidence interval, 1.15 to 7.39), and the overall pattern of associations was similar in sex-stratified analyses. CONCLUSIONS: These findings suggest that LWH independently increases the risk of IHD among Japanese workers, even after adjustment for job strain, underscoring the importance of managing working hours to reduce karoshi in both male and female workers.