Breast Cancer Among Women Who Work at Night

The authors reply: Although most breast cancers appear to be attributable to environmental exposures, 1 little is known about specific causes for this disease. Therefore, it is important to conduct epidemiologic studies in order to test all biological plausible hypotheses. It has been suggested that light-at-night may cause breast cancer and other hormone-related tumours. 2 Despite the high prevalence of persons who work at night and therefore are exposed to light-at-night, no major study has investigated this hypothesis. As an initial step to evaluate an association between night work and female breast cancer, we used a comprehensive data linkage for this purpose, in which it was possible to control for the major confounder, i.e. the reproductive outcome. 3 The strengths of our nationwide register linkage studies are their size and the lack of selection and information bias. 4 A major problem, however, is often the relatively imprecise available information on some exposures. In our attempt to classify workers with predominantly night work, we used the registered information of being employed over half a year in trades in which at least 60% of the women worked at night. Thereby we omitted a major group of hospital workers in which the proportion of night workers is 41%. Owing to our data linkage, the odds ratio for breast cancer among female Danish hospital workers is 1.2 (1.1–1.5), and among the major subgroup, the nurses, the OR is 1.3 (1.1–1.4). Thus, among these groups, which have a lower proportion of night workers than those in our recent study, 3 the increased relative risk of breast cancer further supports our hypothesis. As suggested by O’Connell and Buttimer, women employed in some trades with predominantly nighttime work may also be exposed to electromagnetic or cosmic radiation, which may also contribute to breast cancer risk in our study. Further, confounders such as alcohol, use of oral contraceptives or a lower level of physical activity among the night-time workers may at least partly have contributed to their observed increased risk of breast cancer, and should be considered in subsequent studies. Johnni Hansen

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