Of course most of us can't simulate nature in terms of lighting our tanks, else we'd never see our tank lighted! However, 12 hrs is long enough for any tank lights to be on. In that respect, while we have our lights on/off at various times, at least we are have the duration of light exposure within the ballpark of what reefs in nature are used to--12 hrs. Corals will get used to this if you are consistent with your light. However, sleeping and feeding is necessary, too, and much of this is done when the lights go off for the filter feeding corals.