Why doesnt my light work at night?
First you need to identify if the problem is in the bulb or the fixture. Swap the bulb to another fixture and place a different bulb in this fixture. If the bulb is the problem, replace it.

If the fixture is the problem, I'd start by checking the voltage in the fixture when there's a problem. Then check the voltage at the switch. There could be an arcing wire resulting in the low voltage, and creating a fire risk that you'll want to resolve quickly.

It's also possible that a thermal protection switch inside the fixture is activating and starting to cut power to the light. If that's the issue, and there isn't a problem like insulation covering the fixture, then replacing the fixture is likely the easiest fix.