Light at night and risk of breast cancer: a systematic review and dose-response meta-analysis.

Breast cancer is the most common malignancy in women and the second leading cause of cancer death overall. Besides genetic, reproductive, and hormonal factors involved in disease onset and progression, greater attention has focused recently on the etiologic role of environmental factors, including exposure to artificial lighting such as light-at-night (LAN). We investigated the extent to which LAN, including outdoor and indoor exposure, affects breast cancer risk. We performed a systematic review of epidemiological evidence on the association between LAN exposure and breast cancer risk, using a dose-response meta-analysis to examine the shape of the relation. We retrieved 17 eligible studies through September 13, 2021, including ten cohort and seven case-control studies. In the analysis comparing highest versus lowest LAN exposure, we found a positive association between exposure and disease risk (risk ratio [RR] 1.11, 95% confidence interval-CI 1.07-1.15), with comparable associations in case-control studies (RR 1.14, 95% CI 0.98-1.34) and cohort studies (RR 1.10, 95% CI 1.06-1.15). In stratified analyses, risk was similar for outdoor and indoor LAN exposure, while slightly stronger risks were observed for premenopausal women (premenopausal: RR 1.16, 95% CI 1.04-1.28; postmenopausal: 1.07, 95% CI 1.02-1.13) and for women with estrogen receptor (ER) positive breast cancer (ER + : RR 1.09, 95% CI 1.02-1.17; ER-: RR 1.07, 95% CI 0.92-1.23). The dose-response meta-analysis, performed only in studies investigating outdoor LAN using comparable exposure assessment, showed a linear relation up to 40 nW/cm²/sr after which the curve flattened, especially among premenopausal women. This first assessment of the dose-response relation between LAN and breast cancer supports a positive association in selected subgroups, particularly in premenopausal women.