Infrared thermal cameras typically perform better at night, but this is independent of the brightness and darkness of the surrounding environment.
It is because the ambient temperature at night is much lower than during the day, thermal imaging sensors can display warm areas with high contrast.
Even in relatively cool daytime, the heat energy of the sun can be absorbed by buildings, roads, vegetation, building materials, etc. During the day, various objects absorb heat in the ambient temperature. When using thermal camera sensors to detect, these objects are not significantly different from other warm objects to be detected.
Similarly, if you stay in the dark for several hours (not after sunset), and most thermal imaging cameras will clearly display warm objects. Even during the day, the morning is more pronounced than noon.