Key clinical point: This study shows a positive association between higher outdoor light at night (LAN) exposure and breast cancer risk that appears stronger for estrogen receptor (ER)-positive cancers.

Major finding: The highest vs. lowest quintile of exposure was associated with an increased risk of incident breast cancer (hazard ratio [HR], 1.10; P-trend = .002). This association was stronger for ER-positive breast cancer (HR, 1.12; P-trend = .007) than for ER-negative cancer (HR, 1.07; P-trend = .66).

Study details: This retrospective NIH-AARP Diet and Health Study used 1996 satellite imagery to determine the intensity of outdoor LAN exposure in relation to incident breast cancer in 186,981 postmenopausal women including 12,318 incident postmenopausal breast cancer cases, with more than 16 years of follow-up.

Disclosures: This study was supported, in part, by the Intramural Research Program of the National Cancer Institute (NCI) as well as extramural funding from NCI. The authors declared no conflicts of interest.