Protected areas, such as nature reserves and national parks, are thought to provide a refuge for wildlife, but according to a new study, many of these areas are not safe from light pollution. Thanks to increasing urbanization, many nocturnal skies are no longer dark. Although helpful for humans, artificial night lighting can impact nocturnal wildlife by disrupting natural reproductive cycles, disorienting migratory species, and increasing the risk of predation. To assess how well protected areas shelter wildlife from light pollution and preserve natural darkness, researchers analyzed satellite images of Earth collected at night by the Defense Meteorological Satellite Program between 1992 and 2010. Individual pixels, representative of approximately 3 square kilometers, were assigned a number based on their degree of illumination, ranging from 0 (complete darkness) to 63 (brightly lit urban areas). More than 170,000 unique protected areas were identified using the International Union for Conservation of Nature’s World Database on Protected Areas. The degree of nighttime illumination was then compared with unprotected areas for each continent over the 2 decades. Although 86% of the world’s landmasses remain in relative darkness at night, darkness declined slightly in all regions over the study period. Protected areas were still generally darker than unprotected areas, yet protected areas experienced widespread increases in nighttime light exposure between 1992 and 2010, the team reports online this month in Conservation Biology. In Europe, Asia, and South and Central America, up to 42% of protected areas have experienced significant increases in nighttime lighting. A smaller percentage of protected areas in Europe (24%) and North America (17%) exhibited high levels of nighttime lighting in all years. Based on their findings, researchers propose reduced lighting zones be established around existing refuges to preserve their natural darkness and biodiversity.