In some places around the globe, the lights never go off. Streetlights, roadway lighting, and illuminated signs can deter crime, make roads safer, and enhance landscaping. Undisrupted light, however, comes with ecological, behavioral, and health consequences. In the US, some states have legislation in place to reduce light pollution; however, levels of light at night remain high in many parts of the country. Now, researchers there have investigated correlations between outside nightly light pollution and Alzheimer's disease (AD). "We show that in the US there is a positive association between AD prevalence and exposure to light at night, particularly in...