Funny this has been asked..
My male CANNOT go without the light on at night. The fact that it is pitch black and he can't see anyone or anything freaks him out. He loses ALL color (even in his eyes) and gets very lethargic. He rather enjoys the "company" of his reflection!
I have in there a "dinosaur skull" decoration that takes up most of the bottom of his tank (2.5 gallon) and have it covered in java moss. When he needs some shut eye, he'll disappear in there for awhile since it is dark and cozy for him.
For my community tank and my female, they don't have their lights on at night.