Definition - What does 18/6 mean?
The light cycle of 24 hours light, or lights always on, is only really beneficial for seedling growth, and after the first to second week of the seedling stage, most indoor growers recommend the 18/6 light cycle with 18 hours of light and 6 hours of darkness, the dark time being intended for root growth and a natural resting period for the plants. Going full tilt and doing 24 hours of light is all right, but you may have to add a root stimulator to your medium in order for rooting vigor to be noticeably similar to that of plants grown on an 18 hour light cycle with a period of rest.
MaximumYield explains 18/6
Other reasons for throttling down the plants growth and going to an 18 hour light cycle as opposed to a 24 is heat production. It can get pretty hot in a grow room with improper ventilation and ducting, not to mention fans and possibly an AC compressor on a temp monitor. Another reason to consider 18 hour lighting is the overhead cost of production. Less light hours equals less kilowatt hours run through the meter (if your power supply happens to be metered), and thus lower overhead due to less output spent on keeping the power running.