Light At Night And Breast Cancer Incidence In The Nurses' Health Study II

Abstract

Introduction: Both animal models and observational studies in humans suggest that exposure to light at night (LAN) can disrupt circadian patterns and decrease nocturnal secretion of melatonin, which is hypothesized to disturb the regulation of estrogen and increase breast cancer risk. This study examined the effect of residential outdoor LAN on breast cancer incidence using data from the US-based Nurses' Health Study II cohort. Methods: A total of 110,594 women were followed from 1989-2011. Annual LAN exposure was measured using satellite data from the US Operational Linescan System, which provides time-varying, high-dynamic range data for a composite of lights from cities, towns, and other sites with persistent lighting. These data represent relative levels of nighttime illumination to ~1 km2 scale. We confirmed incident invasive and in situ breast cancer cases by medical record review. We used Cox proportional hazard models to calculate hazard ratios (HRs) and 95% confidence intervals (CIs), adjusting for a number of breast cancer risk factors, including age, race, benign breast disease history, family history of breast cancer, age at menarche, parity, age at first birth, height, BMI, oral contraceptive use, postmenopausal hormone use, smoking status, Alternative Healthy Eating Index, physical activity, marital status, living alone, income, Census tract median home value and median income, and fine particulate matter (PM2.5) air pollution. Results: Over 2,058,737 person-years, we identified 3,196 breast cancer cases. In multivariable models, compared to women in the lowest quintile of LAN, the HRs (95% CIs) were Q2: 1.0 (0.9, 1.1); Q3: 1.1 (0.9, 1.2); Q4: 1.2 (1.1, 1.4); Q5: 1.1 (0.9, 1.2); p for trend=0.03. Results were similar in pre- and post-menopausal women. Conclusions: We observed a positive relationship between residential LAN and breast cancer in this cohort of adult women, indicating that light exposure may play a role in cancer risk.