Shedding light on cancer: An ecological analysis of nighttime illumination and cancer risk

• Ecological study of nighttime illumination and cancer incidence in the Northeastern U.S. • Nighttime light positively associated with hormone-related cancers, especially thyroid. • Inverse association observed between nighttime light and colon cancer. • Results highlight environmental light pollution and disparities in cancer risk. Light at night (LAN) is a widespread environmental exposure and circadian disruptor that may influence cancer risk by interfering with biological and physiological processes. While experimental and epidemiologic evidence links LAN and circadian disruption to hormone-related cancers, large-scale ecological evidence across multiple cancer sites remains limited. We examined the association between ambient LAN and incidence of several cancers, with a focus on hormone-related sites, in the Northeastern United States. This ecological study used county-level data from all seven Northeastern states. LAN exposure was quantified using satellite-derived nighttime illumination, and age-adjusted incidence rates for breast, prostate, thyroid, non-Hodgkin lymphoma (NHL), and colon cancers were obtained from national registries. Linear regression and spatial analyses were conducted, adjusting for population density, poverty, race/ethnicity, insurance, disability, and education. We found significant positive associations between LAN and hormone-related cancers, including breast (p = 0.007), prostate (p = 0.022), non-Hodgkin Lymphoma (p = 0.049), and thyroid cancer (p < 0.001), with the thyroid association being most robust. In contrast, a consistent inverse association was observed between LAN and colon cancer incidence (p < 0.05). These findings suggest that ambient LAN may contribute to higher risks of several hormone-related cancers, potentially modified by socioeconomic and racial/ethnic disparities. Future research using individual-level data and mechanistic studies is needed to clarify pathways linking LAN, circadian disruption, and cancer risk.