About the study

The Nightingale Study is a nationwide prospective cohort study among (former) nurses. The main aim is to investigate effects of occupational exposures on chronic disease risks. Of particular interest is the potential association between shift work and breast cancer risk. Dose-response effects, effects of type, frequency and duration of shift work, and time lag effects will be an important part of the statistical analyses. Biological mechanisms, e.g. on the role of clock genes and melatonin, will also be investigated.

Design and recruitment
In the fall of 2011, 191.971 eligible women (i.e. registered as a nurse in the national healthcare professional registry, aged 18-65 years old, Dutch residency) were invited through the national healthcare professional registry to complete an informed consent form and a web-based questionnaire on occupational exposures, including a detailed questionnaire on all aspects of shift work, and potential confounding factors. The questionnaire was especially designed for this study. The definition of shift work was based on international consensus (Stevens et al Occup Environ Med 2010). The informed consent included consent for linkage with national disease registries, medical record review and use of biospecimens for future research. In addition, at the time of questionnaire completion, women were asked to donate toenails for DNA analyses (e.g. on clock gene SNPs). The cohort will be linked with the Netherlands Cancer Registry and the Nationwide Pathology Database after 5 years to collect information on cancer and with Statistics Netherlands for information on causes of death.
Between October 2011 and February 2012, 59.974 women (31%) responded. Approximately 50% of participants also donated toenails for DNA/SNP analyses. Design considerations, details on the recruitment process and data collection, baseline characteristics, and comparison to similar cohorts will be published in the near future.

Possible results and relevance for (cancer) research
The existing scientific evidence on shift work and breast cancer is scarce. Only few observational studies have been conducted and yielded data that are inconsistent, incomplete, and possibly inaccurate due to recall bias, variations in the definition of shift work, and lack of adjustment for potentially important confounding factors. There is a desperate need for more rigorous scientific research on this topic. Results from the Nightingale Study will contribute to the scientific evidence of potential shift work related health risks and will help develop preventive measures and policy aimed at reducing these risks. The policy implications of an increased risk of cancer in shift workers would be complex because a large and growing proportion of the population must work a non-day shift. The results of our studies may help to answer questions like: should employers warn shift workers about the risks, offer mammographic screening programs, or reorganize shift schedules to minimize short term effects? In addition, this cohort may become a part of international collaborations and will be a source of data and knowledge on the etiology of chronic diseases.

The study was named after Florence Nightingale (1820-1910) to honour her contribution to the nursing profession.

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