You can actually overerlight your corals, heat issues aside.
Regularly timed light cycles benefit the corals, as they don't have to then deal with processing excess food from the zooxanthallae (resulting from uneven light cycles), making them deal with "sugar and oxygen highs" (simply put...
)
I keep my MH on for 10 hours/day, and the actinics on for 12.
Mitch