Night work and breast cancer risk in a general population prospective cohort study in the Netherlands

Experimental studies in animals indicate that disruption of the circadian rhythm is carcinogenic, and night work has been suggested to be a probable breast cancer cause in humans. Findings among humans, however are inconsistent, often gathered with retrospective study designs, and only based on specific populations, such as nurses. We used data on night work collected in the Dutch Labor Force Surveys of 1996 until 2009, and individually linked these with National registers on hospital admission. Among 285,723 women without breast cancer at baseline, 2,531 had a hospital admission for breast cancer during an average of 7 years of follow up in the registers. Occasional and regular night work were not associated with the risk of hospital admission for breast cancer (adjusted hazard ratios 1.04; 95% confidence interval 0.85-1.27, and 0.87; 0.72-1.05, respectively). Working more hours per week, or more years in a job entailing night work did not show increased breast cancer risks. Hazard ratios neither differed between nurses and women with other occupations. Our results show no association of night work with incident breast cancer, and suggest that night work generally does not increase the risk of breast cancer among women in the Dutch working population.