Wegrzyn LR, et al. – This study was meant to assess the links between rotating night–shift work and breast cancer risk, using data from the Nurses' Health Study and Nurses' Health Study II. Findings demonstrated an association between long–term rotating night–shift work and a higher risk of breast cancer, specifically among women who performed shift work during young adulthood.

Methods

• Researchers assessed the associations between rotating night-shift work and breast cancer risk using data from 2 prospective cohort studies, the Nurses' Health Study (1988–2012; n = 78,516) and Nurses' Health Study II (1989–2013; n = 114,559).  

Results

• Data showed that in the 2 cohorts, there were overall 9,541 incident invasive breast malignancies and 24 years of follow-up.
• Researchers found that in the Nurses' Health Study, women with 30 years or more of shift work did not have a higher risk of breast cancer (hazard ratio (HR) = 0.95, 95% confidence interval (95% CI): 0.77, 1.17; P for trend = 0.63) compared with those who never did shift work, although follow-up occurred primarily after retirement from shift work.
• Findings demonstrated that among participants in the Nurses' Health Study II, who were younger than participants in the other cohort, a markedly higher risk of breast cancer was evident in women with 20 years or more of shift work at baseline, reflecting young-adult exposure (HR = 2.15, 95% CI: 1.23, 3.73; P for trend = 0.23), and this risk was marginally significantly higher for women with 20 years or more of cumulative shift work when researchers used updated exposure information (HR = 1.40, 95% CI: 1.00, 1.97; P for trend = 0.74).