i started my auto's at 18 and then decided that 20 is going to work better. But I think that it is good for the plant to have some time of darkness, whether is be 4 or six hours..... i say that only because that it seems when i check on them right after the dark period ( i check my girls about 5 times a day they like when i breath on them ) They seem to have shot up more from the dark period.