Yes. Nighttime is a great time to catch them if they have been buzzing round your house. They tend to hide somewhere when they sleep so they are protected from predators.
Either chase them round the room like a madman, get flypaper or one of those electronic zappers or catch them at night.
I assume you are referring to house files?
Not sleep as we know it. At night they go into a semi dormant state to conserve energy, because it is usually cooler at night.
In the winter most flies perish because they have lived their life cycle - about 7 days after being born from a pupa, depending on the species. The ones born latest survive by crawling into nooks and crevices, again, to conserve energy from the cold.