If you are doing live plants, you want to have the lights on only once during any 24 hour period. You can shorten or lengthen the "day" artificially, but if you try to do two separate periods of light in 24 hours, you will mess up your plants.
Actually this isn't true. Many planted tanks run with a seista time of a couple hours very sucessfully. I've run planted tanks this way. Coralline Algae's advice was on target for the situation.