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.

Background: Different job strain formulations based on the Job Demand-Control model have been developed. This study evaluated longitudinal associations between job strain and psychological distress and whether associations were influenced by six formulations of job strain, including quadrant (original and simplified), subtraction, quotient, logarithm quotient, and quartile based on quotient, in randomly selected Japanese workers. Methods: Data were from waves I and II of the Survey of Midlife in Japan (MIDJA), with a 4-year follow-up period. The study sample consisted of 412 participants working at baseline and had complete data on variables of interest. Associations between job strain at baseline and psychological distress at follow-up were assessed via multivariable linear regression, and results were expressed as β coefficients and 95% confidence intervals including R2 and Akaike information criterion (AIC) evaluation. Results: Crude models revealed that job strain formulations explained 6.93–10.30% of variance. The AIC ranged from 1475.87 to 1489.12. After accounting for sociodemographic and behavioral factors and psychological distress at baseline, fully-adjusted models indicated significant associations between all job strain formulations at baseline and psychological distress at follow-up: original quadrant (β: 1.16, 95% CI: 0.12, 2.21), simplified quadrant (β: 1.01, 95% CI: 0.18, 1.85), subtraction (β: 0.39, 95% CI: 0.09, 0.70), quotient (β: 0.37, 95% CI: 0.08, 0.67), logarithm quotient (β: 0.42, 95% CI: 0.12, 0.72), and quartile based on quotient (β: 1.22, 95% CI: 0.36, 2.08). Conclusion Six job strain formulations showed robust predictive power regarding psychological distress over 4 years among Japanese workers.