Increased breast cancer risk among women who work predominantly at night

Irregular working hours, including working at night, have serious psycholog ical and physiological effects. In a nationwide population-based case-contr ol study, we investigated the breast cancer risk among 30- to 54-year-old D anish women who worked predominantly at night. Individual employment histor ies were reconstructed back to 1964 for each of 7035 women with breast canc er and their individually matched controls from the records of a nationwide pension scheme with compulsory membership. Odds ratios, including 5 years of induction time and adjusted for socio economic status, age at the birth of first and last child and number of children, were estimated by condition al logistic regression analysis. The odds ratio for breast cancer among wom en who worked at night at least half of a year was 1.5 (95% confidence inte rval, 1.2 to 1.7), and there was a tendency to increasing odds ratio by inc reasing duration of nighttime employment.