Then you should probably invest in a timer. You don't have to get one for aquariums. I am pretty sure they have some cheap ones at Walmart and the like. However, if you have low light plants and they are near a window or at least in a well lit room, you might also be fine.
Remember that the tank your fish lives in is his or her's whole world. Ask yourself this question, and answer it honestly: would you choose to live there?