yeah the genetics of a plant are 24 hours a day, with in the 24 hours u can do anythng, but just like having cold temps more then 24 hours is just not what a plant is genetically encoded to destiguish, it will probably be fine and grow but you will most likely run into a hermie or somethin. Im at 18/6 and it seems to be perfect, i find that anything more then 20 hours on is a waste of electricity, the roots grow on the dark periods the most.