Turn your lights off at night. There are a couple of reasons for this. For one, your fish do sleep, some will float in one spot while others will actually just lay on the bottom and sleep.(for instance my mandarin sleeps in the same corner of my tank every night, and turns a light white color while he sleeps, then regains color in the morning.)
The whole idea of having an aquarium is to recreate their natural environment. Its not sunny 24 hours a day in nature. So having the lights on all the time will stress your fish out and most likely lead to a shorter life. Its best to have your lights on a timer so its the same amount of light everyday allowing them to get used to the cycle.
Another great reason to turn your lights off at night is algae. Algae needs light to grow. The more light, the more algae you will grow. By having your lights off part of the time, you will reduce the amount of algae growth so your tank wont get dirty as fast.
If you have any nocturnal fish you could get something like this which will simulate the moon light so you can watch your nocturnal fish, while still allowing your other fish to sleep.