Breast cancer and night work among Norwegian nurses

Objective Previous studies have suggested an association between breast cancer and night work. We evaluated the relationship among Norwegian nurses. Methods A case-control study, nested within a cohort of 44,835 nurses educated between 1914 and 1980 was performed, based on a registry of all Norwegian nurses. Four controls were individually matched by year of birth to each of 537 breast cancer cases that occurred during the period 1960-1982. The reconstruction of work history and number of years with night work for each nurse was based on information from the nurse registry, and data from three censuses. We used conditional logistic regression to calculate odds ratios (ORs) and 95% confidence intervals (CIs), adjusted for total duration of work as a nurse and parity. All statistical tests were two-sided. Results The adjusted OR of breast cancer among nurses who worked nights for 30 or more years was 2.21 (CI 1.10-4.45) compared with those who did not work nights after graduation from nursing school (p(trend) = 0.01). Conclusion Our results are in accordance with previous studies that find an association between night work and breast cancer risk among women.