Well...Leaving my lights on for 24/7 appeared to be the beginning point of my happy, healthy marine tank turning into a bleached, wasteland deathtrap... Changed the timer because the time was wrong then went on vacation for 3 weeks. When I came back half the corals were bleached or dead, the other half were retracted, several fish were dead. I never was able to get any fish or coral to grow well after that and most died within a few weeks of being put in.
So...Anecdotally, I'd say don't leave the lights on 24/7