Yes, they do sleep. Most people keep their tank lights on for 10-12 hrs a day; easiest to get a cheap timer so you don't have to remember it.
It is a good idea to let the room lights come on a little before the tank lights and stay on just a little bit longer to make it less of a shock to the fish. (Having said that, night falls quickly in the tropics).
Of course some species are nocturnal and spend the day snoozing in a cave or under a plant. My corys seem to be crepuscular, I find most of the action happens at dawn and dusk (ie between room lights and tank lights), so that's when I try to draw up a chair and watch.
If you keep the lights on for too long you are likely to have problems with algae growth. Too little light and your plants won't survive. Anyway, I think fish appreciate the change between night and day.
If you sneak up on your fish after lights out you should be able to see them stand in a corner asleep. This is also the time my livebearers choose to give birth.
so thank you for the info - my fish thank you too 
