Why can the same headlights seem brighter at night than during the day?

Answer: Darker surroundings increase contrast, and our eyes adapt to lower light

Darker surroundings increase contrast, and our eyes adapt to lower light ✓ — The same light stands out more against a dark background, while adaptation to low light changes our visual sensitivity. Both contribute to how bright a headlight appears; neither requires the lamp to produce more light.

Night air concentrates the light into a stronger beam — Night air does not normally focus headlights into a stronger beam. The change is largely in the viewing conditions and our visual response, rather than air concentrating the light.

The bulbs automatically produce extra light when their surroundings darken — Dark surroundings do not by themselves make a bulb emit more light. Compare the same headlights at the same setting: their apparent brightness can change even when their output stays constant.