To address weiwei_uk's question, rats mice etc only come out at night so they tend to catch more that way. As far as I know cats don't hunt more any particular time of day.

As for safety, they're far safer at night when the roads are quiet and there's fewer people and predators around.

I'll add a rider to that, we live in a relatively quiet urban area with no real cat predators and little traffic at night. In other parts of UK, let alone the world it may well be a different situation.