Working hours as a risk factor for acute myocardial infarction in Japan: case-control study

Objective: To clarify the extent to which working hours affect the risk of acute myocardial infarction, independent of established risk factors and occupational conditions. Design: Case-control study. Setting: University and general hospitals and routine medical examinations at workplaces in Japan. Subjects: Cases were 195 men aged 30-69 years admitted to hospital with acute myocardial infarction during 1990-3, Controls were 331 men matched at group level fur age and occupations who were judged to be free of coronary heart diseases at routine medical examinations in the workplace. Main outcome measures: Odds ratios for myocardial infarction in relation to previous mean daily working hours in a month and changes in mean working hours during previous year. Results: Compared with men with mean working hours of >7-9 hours, the odds ratio of acute myocardial infarction (adjusted for age and occupation) for men with working hours of >11 hours was 2.44 (95% confidence interval 1.26 to 4.73) and for men with working hours of less than or equal to 7 hours was 3.07 (1.77 to 5.32). Compared with men who experienced an increase of less than or equal to 1 hour in mean working hours, the adjusted odds ratio of myocardial infarction for men who experienced an increase of > 3 hours was 2.53 (1.34 to 4.77). No appreciable change was observed when odds rations were adjusted for established and psychosocial risk factors for myocardial infarction. Conclusion: there was a U shaped relation between the mean working hours and the risk of acute myocardial infarction. There also seemed to be a trend for the risk of infarction to increase with greater increases in mean working hours.