A 12 year prospective study of circulatory disease among Danish shift workers

Background: Previous studies of the risk of heart disease after shift work reached different estimates and review authors disagree about the validity of some of the studies. A cross sectional study showed that shift workers had a higher prevalence of nearly every unfavourable work environment factor investigated. Conflicts at work and low decision latitude were more frequent among shift workers, and all-day walking or standing work and part-time jobs were more often found among female shift workers. Objectives: To estimate the risk of circulatory disease in a prospective follow up of a representative sample of gainfully employed Danes, considering known or suspected confounding factors. Methods: A cohort of 5517 people who were gainfully employed in 1990 were followed up for all hospital treatments due to circulatory diseases ( 390 - 458, ICD-8; 100 - 199, ICD-10) from 1991 to 2002 inclusive. A log linear Poisson regression model was applied to control confounding factors and calculate the relative risk for 927 men and women working nights, evenings, or other non-day shifts compared to 4579 day workers. Results: Non-day workers compared to day workers had a relative risk (RR) for all circulatory diseases of 1.31 ( 95% Cl 1.06 - 1.63). Without control for BMI and smoking, the RR estimate was 1.33 ( 95% CI 1.07 - 1.65). For a subgroup of workers with at least three years' seniority, the RR was 1.40 ( 95% CI 1.09 - 1.81). The population based aetiological fraction of shift work was estimated to 5%. Conclusion: This study adds to a growing body of evidence suggesting that shift work carries an excess risk of circulatory diseases.