The dark side of light at night: assessing breast cancer risk with a dose-response meta-analysis

Light at night (LAN) is a ubiquitous environmental issue resulting from exposure to artificial light during the night-time hours. The adverse effects of LAN include circadian disruption, melatonin suppression and altered clock gene expression. Very recent evidence suggests that LAN may increase risk of several diseases, including cancer. Breast cancer is the most common diagnosed cancer globally and the second leading cause of cancer-related mortality. Several environmental and life-style related risk factors have been linked to breast cancer onset. In this updated meta-analysis, we investigated how exposure to LAN may affect breast cancer risk. Using systematic online database searches (PubMed, Scopus, and Web of Science), we included 19 eligible studies (10 cohort and 9 case-control) investigating the association between LAN, either outdoor or indoor, and breast cancer risk. We first performed a meta-analysis comparing the highest versus lowest levels of exposure. Then, we performed a dose-response meta-analysis to investigate the shape of such relation. Overall, we found a positive association between LAN and breast cancer (risk ratio [RR] 1.08, 95% confidence interval-CI 1.03-1.13). In stratified analyses, the risk was more pronounced in premenopausal (RR 1.15, 95% CI 1.05-1.26) and normal-weighted (RR 1.17, 95% CI 1.00-1.36) women, while stratifying for outdoor/indoor exposure and for estrogen receptor status did not gave substantial differences in the risk estimates. The dose-response relation showed a linear risk increase up to 60 nW/cm(2)/sr after which the curve reached a plateau, especially among premenopausal women. These results strengthen the hypothesis that LAN is associated with breast cancer risk. Public health measures are warranted to minimize LAN exposure and reduce its related adverse effects on human health. KEY MESSAGES: • Light at night is positively associated with breast cancer risk. • The dose–response relation between light at night and breast cancer supports a positive association particularly in premenopausal women.