(ChemotherapyAdvisor) – Women who perform night shift work are at risk for developing breast cancer, according to researchers of the Institute of Cancer Epidemiology, Danish Cancer Society, Copenhagen, Denmark. This conclusion is based on a study entitled “Nested Case-Control Study of Night Shift Work and Breast Cancer Risk among Women in the Danish Military,” which was published online in Occupational & Environmental Medicine on May 29.

Based on previous studies that had suggested a link between night shift work and breast cancer, the investigators conducted a nationwide case–control study containing 18,551 female military employees of Denmark. The aim of the study was to investigate the risk for breast cancer after a period of night shift work, as well as to explore the role of leisure time, sun exposure, and diurnal preference in that risk.

Overall, the investigators observed an adjusted overall risk (OR) of 1.4 (95% CI 0.9–2.1) among women who had never performed night shift work. The relative risk (RR) for breast cancer tended to increase with increasing number of years of night shift work (P=0.03). The most pronounced effect of night shift work on breast cancer risk was observed in women with intense night shifts (OR=3.9, 95% CI 1.6–9.5). Night shift workers tended to sunbathe more frequently than day workers.

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The investigators concluded: “The results indicate that frequent night shift work increases the risk for breast cancer and suggest a higher risk with longer duration of intense night shifts.”