Light at Night and Breast Cancer Risk Among California Teachers

Background: There is convincing evidence that circadian disruption mediated by exposure to light at night promotes mammary carcinogenesis in rodents. The role that light at night plays in human breast cancer etiology remains unknown. We evaluated the relationship between estimates of indoor and outdoor light at night and the risk of breast cancer among members of the California Teachers Study. Methods: Indoor light-at-night estimates were based on questionnaire data regarding sleep habits and use of nighttime lighting while sleeping. Estimates of outdoor light at night were derived from imagery data obtained from the US Defense Meteorological Satellite Program assigned to geocoded addresses of study participants. Analyses were conducted among 106,731 California Teachers Study members who lived in California, had no prior history of breast cancer, and provided information on lighting while sleeping. Five thousand ninety-five cases of invasive breast cancer diagnosed 1995–2010 were identified via linkage to the California Cancer Registry. We used age-stratified Cox proportional hazard models to calculate hazard ratios (HRs) with 95% confidence intervals (CIs), adjusting for breast cancer risk factors and neighborhood urbanization and socioeconomic class. Results: An increased risk was found for women living in areas with the highest quintile of outdoor light-at-night exposure estimates (HR = 1.12 [95% CI = 1.00–1.26]; test for trend, P = 0.06). Although more pronounced among premenopausal women (HR = 1.34 [95% CI = 1.07–1.69]; test for trend, P = 0.04), the associations did not differ statistically by menopausal status (test for interaction, P = 0.34). Conclusions: Women living in areas with high levels of ambient light at night may be at an increased risk of breast cancer. Future studies that integrate quantitative measurements of indoor and outdoor light at night are warranted.