What sort of corals do you have? If you have SPS, you may need a longer light cycle to keep the colors up.
From what I've read, corals can only use 6 hours of light...the rest is just wasted. If you wanted your corals to just grow and grow, you would do 6 hours lights on, 6 hours lights off, repeat.
After 6 hours, as long as you're not having algae or bleaching issues, it's mostly just for our enjoyment.