You could try a light with a dimmer. In the wild the nights are seldom completely dark so they must act naturally if the light goes down to the level of a full moon. You'd still be able to see them. It is best tried at the time that you normally turn the lights off at night, as fish also have a timeclock.
A long time ago a friend had a beautifully designed setup in a 24 gallon tank made up to look like an underground cave. The lighting was very dim and ofcourse, the tank was stocked with a half dozen blind cave fish (Astyanax mexicanus). It was noted that late at night the fish would still rest on the gravel or a rock ledge.