Association between working hours and cancer risk in Japan: The Japan public health center‐based prospective study

OBJECTIVES: In this study, we evaluated the association between working hours and cancer risk in the Japanese population, which has not been evaluated. METHODS: Using a cohort database from a Japan Public Health Center‐based Prospective Study, we evaluated 26 738 participants (16 351 men and 10 387 women), who responded to a questionnaire about working hours and followed these participants from 1993–1994 to 2013. Participants were divided into four groups according to working hours (≤6, 7–8, 9–10, ≥11 h/day). The hazard ratio (HR) and 95% confidence interval (CI) of each cancer incidence were calculated using a multivariable‐adjusted Cox proportional hazard model. RESULTS: During 488 383 person‐years of follow‐up, 481 patients with newly diagnosed cancers were identified. There was no clear association between long working hours and overall cancer, lung cancer, and stomach cancer risks. Long working hours tended to increase prostate cancer risk in men and breast cancer risk in women, although the difference was not statistically significant. Increased liver cancer risk with short working hours (HR [95% CI]; 3.15 [1.44–6.88] in the ≤6 h/day group vs. 7–8 h/day) was observed. Colorectal cancer also tended to increase risk in short working hours, however, there were not statistically significance. CONCLUSIONS: In this population, long working hours were not associated with cancer risk with statistically significance. The association between short working hours and liver cancer risk was observed, probably due to the reverse causation of liver cancer.