BACKGROUND: Outdoor artificial light at night (ALAN) may increase breast cancer risk by suppressing melatonin secretion, an effect influenced by light intensity and wavelength.
OBJECTIVES: We evaluated the association of multiple ALAN measures with breast cancer risk.
METHODS: We pooled data from two cohort studies (baseline: 2009-2016; n = 24,793 female participants, 674 breast cancer cases). ALAN exposures at residential addresses were estimated using images taken from the International Space Station (2011-2013; ∼30-meter spatial resolution). Measures included average visual radiance (i.e., brightness), average melatonin suppression index (MSI), which quantifies the extent to which light suppresses melatonin, and their product (average impact MSI). Cox proportional hazards regression, adjusted for breast cancer risk factors and built environment features, was used to generate hazard ratios with 95 % confidence intervals (CI) for the associations between ALAN and breast cancer incidence. We also explored associations with breast cancer subtypes (invasive ductal, luminal A, postmenopausal) and among participants self-reporting light entering their rooms while sleeping.
RESULTS: No statistically significant associations were observed in the overall study population. Among women reporting light entering their rooms, statistically significant associations of average impact MSI with breast cancer risk, overall and across each subtype, were observed. For example, those in the highest versus lowest tertile of average impact MSI had a 1.53-fold increased hazard of overall breast cancer (95 % CI: 1.18-1.98).
DISCUSSION: Our findings suggest that outdoor ALAN is associated with increased risk of breast cancer and that both intensity and wavelength of light should be considered when evaluating ALAN exposures.
