Take some time and watch your fish from a distance when the tank lights are both on and off. You’ll notice a difference in behavior. My fish, both mbuna and Tanganyikans, are generally less erratic during the day when the lights are off. I’ve mentioned before, my mbuna experiences suggest that things turn sinister when there is complete darkness. But then again, many mbuna species can be pretty nasty if you don’t have the right tank set-up and the right gender mixes.
Unless you’re keeping plants, there is no real need to keep the lights on for long periods of time. By doing so, you’re inviting algae problems and probably stressing your fish out more than is necessary. Experiences vary, but lights aren’t on any of my tanks longer than a couple of hours a day. My house gets considerable ambient light during the day (not directly on the tanks) such that keeping my LEDs on isn’t necessary.