A scientific report on light pollution opens by defining skyglow as the brightening of night skies over cities. It then classifies major sources into streetlights, commercial signage, and outdoor sports facilities. In the next section, it explains effects on different groups: migratory birds become disoriented, insects cluster around lights, and human sleep can be disrupted. The report uses the phrase 'these sources' to link the classification section to the effects section, reminding readers that the impacts trace back to the same categories.