I'm struggling to grow under 4, 4'flouro tubes all the way thru (cool blues at that, yeah I'm a masochist) but I used a 18/6 and vegged for 2 months (like I said I used tubes. lol) but the growth was fantastic undertubed flouros on a 18/6 i agree, plants do their goodies at noght, during the day they absorb energy andd just grow upwards. at night they produce the good stuff inside so a little rest never hurt anyone.