Oh it is most definitely not less power more hours. SM has covered this many times and I believe it is in the FAQ. When he came out with the "double the light, half the hours" method to battle the black growth (which turned into a general rule to get better green growth) he covered this. It's all about intensity of the light. More intense light for a shorter period will promote green growth better, and green growth filters better. Less intense light for a longer period will not grow green nearly as well. Not to mention that this rule generally applied to people already running 18 on / 6 off, so further shortening the dark recovery period would do more harm than good.