A cohort study on the_effect of shift work on the onset of circulatory diseases

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Objectives ; To investigate the effect of shift work on the onset of circulatory diseases, we took out a cohort study among the physical workers in a factory. The number of subjects was 2,702.Method ; We followed them from 1987 to 1997. The subjects were grouped into three groups by the type of work schedule in 1987, the baseline year, "day workers", "shift workers without midnight shift, and "shift workers with midnight shift".Results ; The onset number of vascular events during 10 years was 18 for all, 2 cerebral hemorrhage, 9 cerebral infarction, one subarachnoidal hemorrhage, arid 5 myocardial infarction. The cumulative incidence of "shift workers with midnight shift aged 50-59 years was higher than those of other two groups. However, the hazard ratio of the group with midnight shift for the onset of circulatory diseases adjusted for age, mean blood pressure, and BMI was not significantly high by the Cox's proportional hazard analysis. The onset, of hypertension was 87 from 1964 subjects. The incidence rate of hypertension was 4.9%. Among the subjects younger than 30 years old, the incidence rate was 2.3% in "day workers", 3.4% in "shift workers without midnight shift, and 6.5% in "shift workers with midnight shift". There was no significant difference between the groups by work schedule among thirties and forties. The hazard ratio of shift workers with midnight shift compared to day workers was significantly high, 1.6, by the Coxs proportional hazard model. That of shift workers without midnight shift was as sane as that of day workers. Conclusions ; It is suggested that night and shift work is a risk factor of hypertension. However we could not find the relationship between the night and shift work and circulatory diseases, such as cerebrovascular disease and myocardial infarction. This negative data might be caused from the health administration such as hypertension control in this factory.