The mitzvah of checking for chametz occurs on the evening before Pesach. I can't find the source now (erev Pesach and I'm in a hurry), but my recollection is that the reason for checking at night by candlelight, rather than during the day, is that it is easier to find chametz at night than during the day.
This seems totally counterintuitive. Obviously, you would think, you can see better during the day so checking should be done then. Isn't it totally backwards to think you could see better at night?
I think the reasoning goes as follows. Nowadays we have two things that ancient people did not: modern building techniques and electricity. Once upon a time most people lived in closely spaced stone huts with small if any windows. Thus, most of their indoor areas were necessarily rather dark. Without electricity, the only way of lighting these areas was with candles.
So checking for chametz by candlelight made perfect sense: it meant using the most efficient possible indoor lighting. But why at nighttime? If most indoor areas received little light from outside, daylight would do little to help the checking. But it would interfere with checking, for the following reason. People's eyesight adjusts to the amount of light they perceive, and even a person who did bedikat chametz inside would probably go outside the building every so often. Upon reentering the home, everything would appear dark even with the available indoor lighting. But at night, it would be just as dark outside as inside. People could check the corners of their houses in the most effective way – with all the available indoor lighting (candles), and with the sensitivity in vision that was unavailable during the day.