A new prospective cohort study (the Nightingale Study) among nurses in the Netherlands was initiated in collaboration with the Institute for Risk Assessment Sciences (IRAS, Utrecht University) and the nationwide register of healthcare professionals (BIG-register of the Ministry of Health, Welfare and Sport, The Hague). Exposure to light-at-night has been suggested as a contributing cause of breast cancer (IARC classification “probable human carcinogen, 2A”). Since shift- and night-time work is prevalent and increasing in modern societies, this exposure may contribute to the continuing elevation in breast cancer risk and may be of public health concern. This study will provide insight into, among others, the potential association between occupational exposures (eg, shift work, electromagnetic fields) and the risk of cancer and other diseases, and on potential biological mechanisms. The total projected study population is 50 000 women. Through the BIG-register, women with a nurse diploma and who are younger than 60 years, will be asked to complete a (web-based or paper) questionnaire, sign an informed consent, and donate toenails for DNA analyses (eg, clock genes). In the spring of 2011, a pilot study (N=800) will be conducted of which the results will be presented. Specific attention will be directed to potential shift-work related selection.