Light at night and breast cancer risk: results from a population-based case-control study in Connecticut, USA

Objective To investigate the potential association between domestic exposure to light at night (LAN) and the risk of human breast cancer. Methods A case-control study of female breast cancer was conducted in Connecticut. A total of 363 incident breast cancer cases and 356 age frequency-matched controls were interviewed using a standardized, structured questionnaire to obtain information on sleeping patterns and bedroom light environment. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated by unconditional multivariate logistic regression. Results A non-significantly increased risk of breast cancer was observed among postmenopausal women for those keeping lights on while sleeping (OR = 1.4, 95% CI 0.7, 2.7), those who reported mainly sleeping in the daytime (OR = 1.4, 95% CI 0.5, 4.3), and those not drawing the curtains/window shades while sleeping at night (OR = 1.2, 95% CI 0.8, 1.9). Conclusion The results from this study suggest a potential increased risk of breast cancer associated with domestic exposure to LAN. Further studies with larger sample size are needed to confirm the results.