I don't think there is a hard and fast rule regarding this, but it seems the preferred method is this...
Light your sump/DSB on a reverse cycle to your other lights, in other words, when your display lights go off then your sump lights come on, this assists in keeping your PH level more stable, you don't get too much of a drop in PH at night.
Some aquarists light their DSB's 24/7, some do it the same as their tank lights.