Nighttime warming dominates the thermal evolution of lakes across China

Diurnal asymmetric warming profoundly affects lake thermodynamics and aquatic ecosystems, yet its large-scale patterns and drivers remain poorly understood. Here we analyze diurnal surface temperature trends of 613 lakes across China from 2001 to 2025, disentangling climatic and anthropogenic contributions. Nighttime warming drives long-term lake warming nationally, with 84.01% of lakes showing a nighttime-dominant pattern. Nighttime lake surface water temperature (LSWT) rose at +0.34 °C decade^-1, while daytime LSWT cooled at -0.24 °C decade^-1. Daytime LSWT is primarily controlled by air temperature, whereas nighttime LSWT is dominated by downwelling longwave radiation, with human footprint exerting a persistent influence across the diurnal cycle. Lake area expansion correlates negatively with daytime warming and positively with nighttime warming nationwide, while anthropogenic activity further amplifies this asymmetry, most strongly in the Eastern Plain lakes. These findings highlight nighttime warming as the dominant force in Chinese lake thermal evolution, informing future conservation and management.