if u feel u need a break...like maybe for temp. or other reasons (this is a highly debated topic)...either would work...personally i like 24/7 but am experimenting with 22/2 at the moment in my grow cab. just to see what happens...i'd go with 20/4 if your looking for a light break but i'm sure many others will say 18/6...experiment and see what works best...personally my plants grow best with 24/7 lighting as long as they are properly ventilated and kept in the optimal temp. range 70-76 F...Ed Rosenthal is the guru of ganga in my opinion and he claims every hour of darkness is an hour of potential growth (photosynthesis) that is lost...but as i said its a hot topic