Working hours and ischaemic heart disease in Danish men: a 4-year cohort study of hospitalization.

Four groups of men with non-daytime work were identified in two surveys. The relative risk of being admitted to hospital due to ischaemic heart disease (IHD) (ICD-8, 410-414) was measured in a 4-year cohort study of all 1,293,888 economically active men in Denmark, aged 20-59 years. Compared with occupational groups having day-work only, men in occupations with frequent night and early morning work had an excess standardized hospitalization ratio (SHR) of 193, occupational groups with late evening work had an excess SHR of 216, groups working in rosters covering 24-hour services had an excess SHR of 174, and groups having other irregular working hours had an excess SHR of 172. We conclude that night work rather than shifts is responsible for a raised risk of IHD. More research is needed to develop preventive strategies.