Outdoor light at night and breast cancer risk after menopause in the NIH-AARP diet and health study

Women in the highest quintile of outdoor light at night exposure had a 10% increase in postmenopausal breast cancer risk compared to those in the lowest quintile.

• The study followed 186,981 postmenopausal women over approximately 16 years, identifying 12,318 cases of breast cancer.
• A 10% increase in risk was observed for postmenopausal breast cancer with higher outdoor light at night exposure (hazard ratio 1.10).
• The association was stronger for estrogen receptor positive breast cancer, with a hazard ratio of 1.12 compared to 1.07 for estrogen receptor negative cancer.
• Individual factors like smoking, alcohol consumption, sleep duration, BMI, and neighborhood environment may influence the relationship between light exposure and breast cancer risk.

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