1. Light pollution is increasingly affecting biodiversity and may also disrupt seasonal adaptations. Even dim artificial light, such as skyglow—which can spread far beyond urban areas—can interfere with using photoperiod as a seasonal cue.

2. Here, we test how light pollution impacts diapause induction and whether urban evolution counteracts it, by using common-garden experiments with a common, widespread geometrid moth (Chiasmia clathrata). We raised offspring from urban and rural populations from North- and Mid-European countries in treatments with and without dim light at night.

3. The dim light treatment strongly increased direct development overall—with no evidence for urban adaptation to it—but distinctly more so in Mid- than in North-European populations.

4. Because diapause induction is critical for surviving winter, these results indicate that dim but widespread light pollution may have detrimental effects on insect populations, especially so at mid-latitudes, and may hence explain part of the ongoing, large-scale insect declines globally.

5. Synthesis and applications: Latitudinal variation in sensitivity to light pollution means that its contribution to insect declines—and its conservation importance—should likewise vary among regions. In mid-latitude regions, where populations seem more sensitive to light pollution, mitigating light pollution should be a high priority for insect conservation Reducing skyglow from cities should benefit both urban populations—which were just as susceptible as rural populations in our study—and rural populations—because skyglow extends far beyond the geographic boundaries of cities.