A weakened diurnal weather constraint leads to longer burning hours in North America.

Contemporary North American wildfires exhibit increasingly erratic intraday burning, posing immediate operational and socioeconomic challenges. Here, we show that climate-driven weakening of day-night (diurnal) weather constraints extends and intensifies burning hours, a key mechanism behind broader fire regime transformations. Analyzing hourly geostationary satellite observations for ~9000 fires (>200 hectares; 2017-2023), we found western mountains and boreal forests experienced the longest active burning hours, with approximately one-third of active days exceeding 12 hours. About 60% of fires reached peak intensity within 24 hours of detection, while 14% of active days peaked at night. On the basis of fire weather, annual potential burning hours were estimated to rise 36% over 1975-2024, with pronounced increases in western regions and spring/fall (48 to 57%). Regions with significant changes gained 26 more potential active days annually and 1.2 additional potential burning hours daily, while extreme days (≥12 or 24 potential burning hours) rose 81 to 233% in fire-prone biomes. Future management requires adaptation to wildfires that increasingly defy diurnal norms.