Night-time satellite images have shown that cities are getting brighter, and light pollution is now recognized as an environmental issue. But how should local governments even begin the work of resolving this if nobody really knows what the sources of all this light are? A large citizen science project led by Dr. Christopher Kyba, at the time geographer at Ruhr University Bochum and the GFZ Helmholtz Centre for Geosciences, is now shedding light on the matter. In 2021, 257 participants examined a total of 234,044 lights over 3,868 individual measurements via an app. The result: More than one light per person in Germany is still on late at night, and streetlights are in the minority. The research team published their results in the journal Nature Cities on June 16, 2025.

“Our participants created a complete lighting survey for public spaces with a total surface area of about 22 square kilometers,” Kyba says proudly. After co-developing the “Nachtlichter” app (“nightlights”), they counted and classified almost a quarter million light sources. In addition to the type of lighting, they documented additional information such as the level of shielding or size of the source. The counting campaigns took place in 33 municipalities in pre-defined areas that match the footprint of a night-time light observation satellite. The areas included city centers, residential neighborhoods, and urban locations that are commercial or industrial in character.

“By comparing these data with the satellite observations, we identified a correlation between the number of counted lights per square kilometer and the radiance observed by the satellite sensor,” says Kyba. “By scaling our results up to cover all of Germany, we estimate that just over one light per person stays on after midnight.” Streetlights are in the minority: In heavily developed areas, there is about one illuminated sign and one illuminated shop window for each streetlight. The group also learned that private lighting is very important. Private windows were the most frequently observed light source by far, even after midnight. Other light sources, such as bright floodlights and illuminated doorbells to decorative lights in yards and gardens, made up about a quarter of the lights observed.

“These results show significant potential for future light and energy savings in German municipalities,” says Kyba. “Both energy and lighting policy as well as research on the effects of artificial light on the environment have generally focused on street lighting. Our findings indicate that a broader approach that considers all lighting is necessary in order to understand and reduce the environmental impacts of light in cities.”