Outdoor Light at Night and Breast Cancer Incidence in the Nurses’ Health Study II

BACKGROUND: Animal and epidemiologic studies suggest that exposure to light at night (LAN) may disrupt circadian patterns and decrease nocturnal secretion of melatonin, which may disturb estrogen regulation, leading to increased breast cancer risk. OBJECTIVES: We examined the association between residential outdoor LAN and breast cancer incidence using data from the nationwide U.S.-based Nurses’ Health Study II cohort. METHODS: We followed 109,672 women from 1989 through 2013. Cumulative LAN exposure was estimated using time-varying satellite data for a composite of persistent nighttime illumination at [Formula: see text] scale for each residence during follow-up. Incident invasive breast cancer cases were confirmed by medical record review. We used Cox proportional hazard models to calculate hazard ratios (HRs) and 95% confidence intervals (CIs), adjusting for anthropometric, reproductive, lifestyle, and socioeconomic risk factors. RESULTS: Over 2,187,425 person-years, we identified 3,549 incident breast cancer cases. Based on a fully adjusted model, the estimated HR for incident breast cancer with an interquartile range (IQR) ([Formula: see text]) increase in cumulative average outdoor LAN was 1.05 (95% CI: 1.00, 1.11). An association between LAN and breast cancer appeared to be limited to women who were premenopausal at the time of a case [[Formula: see text] (95% CI: 1.01, 1.14) based on 1,973 cases vs. [Formula: see text] (95% CI: 0.91, 1.09) based on 1,172 cases in postmenopausal women; [Formula: see text]]. The LAN–breast cancer association was observed only in past and current smokers at the end of follow-up [[Formula: see text] (95% CI: 0.94, 1.07) based on 2,215 cases in never smokers; [Formula: see text] (95% CI: 1.01, 1.19) based on 1,034 cases in past smokers vs. [Formula: see text] (95% CI: 1.07, 1.37) for 300 cases in current smokers; [Formula: see text]]. CONCLUSIONS: Although further work is required to confirm our results and to clarify potential mechanisms, our findings suggest that exposure to residential outdoor light at night may contribute to invasive breast cancer risk. https://doi.org/10.1289/EHP935