People who use an 18/6 cycle claim that pot plants need to "sleep". I have not found this to be true. I've always used a 24/0 cycle indoors for veg, and compared my pot with people who ran an 18/6 cycle. Cuttings from the same donor plant, identical medium, nutes, HPS wattage, etc. If there was a significant difference in the pot I couldn't tell. The main difference is my pot finished faster.
However I have no problem with people who use an 18/6 or 16/8 cycle.
I knew someone who did an experiment a few years back. Since the sun's cycle changes daily, he gradually reduced his light cycle a few minutes/day. The results? No difference except it took longer for him to get a finished product.