Night-shift work and risk of breast cancer in Korean women.

Purpose: This study evaluated the association between night-shift work (NSW) and breast cancer risk as well as subtypes of breast cancer in Korean women. Patients and methods: The study population included 1721 female breast cancer cases and 1721 female controls matched by age. The subtypes of breast cancer were determined based on estrogen, progesterone, and human epidermal growth factor receptor 2 statuses by reviewing pathology reports. Odds ratios (ORs) for NSW experience, age at commencement of NSW, frequency, and duration were estimated using conditional logistic regression and were adjusted for confounders such as parity and socioeconomic status-related factors. Results: Among 1721 pairs, 10.58% of cases and 9.59% of controls had ever engaged in NSW. NSW was not associated with breast cancer risk in terms of ever having night-shift exposure (adjusted OR was 1.11, 95% confidence interval [CI] =0.89-1.40), duration, frequency, or cumulative working time. The OR for >10 years of lifetime duration of NSW was 1.55 (95% CI 0.89-2.69, P=0.124). In addition, the OR for >35,000 hrs for cumulative frequency of night work was OR=1.42 (95% CI=0.73-2.74, P=0.304). There was no heterogeneity in ORs of ever having NSW and cumulative duration of NSW between four subtypes of breast cancer. Conclusion: NSW including long-term and heavy working exposure was not associated with breast cancer risk.