I don't think you really need the 6 hours of darkness, considering you're watering the plants, giving them nutrients, and controlling the light & temp in the room. Most proponents of the 18/6 cycle will say that during the 18 hours of light, your plants are processing the light via photosynthesis and the 6 hours of darkness allow for root growth. Considering you're controlling the environment though, root growth isn't really a huge factor unless you're growing plants so huge that they could unroot themselves from being top heavy.