Objectives The deleterious health effects of long working hours have been previously investigated, but there is a dearth of studies on mortality resulting from accidents or suicide. This prospective study aims to examine the association between working hours and external-cause mortality (accidents and suicide) in Korea, a country with some of the longest working hours in the world.

Methods Employed workers (N=14 484) participating in the Korean National Health and Nutrition Examination Survey (KNHANES) were matched with the Korea National Statistical Office’s death registry from 2007–2016 (person-years = 81 927.5 years, mean weighted follow-up duration = 5.7 years). Hazard ratios (HR) for accident (N=25) and suicide (N=27) mortality were estimated according to weekly working hours, with 35–44 hours per week as the reference.

Results Individuals working 45–52 hours per week had higher risk of total external cause mortality compared to those working 35–44 hours per week [HR 2.79, 95% confidence interval (CI) 1.22–6.40], adjusting for sex, age, household income, education, occupation, and depressive symptoms. Among the external causes of death, suicide risk was higher (HR 3.89, 95% CI 1.06–14.29) for working 45–52 hours per week compared to working 35–44 hours per week. Working >52 hours per week also showed increased risk for suicide (HR 3.74, 95% CI 1.03–13.64). No statistically significant associations were found for accident mortality.

Conclusions Long working hours are associated with higher suicide mortality rates in Korea.

Key terms accident; depression; injury; karoshi; KNHANES; Korea; Korean National Health and Nutrition Examination Survey; long working hour; mental health; occupational; overwork; suicide; work hour; working hour; working time; worktime