I have read in hightimes Q&A that plants will not photosynthisize more than 18 hrs a day... I have also heard from local experienced growers that by keeping the lights on 22-24 hrs a day you prevent stretch(distance between nodes or leafsets)...that said... I personnally go 20 on 4 off...for no other good reason than to give time for the lights to coo and give em a break..... about all I got...peace