I have always used 24h right from seed.
I have seen no evidence that points to stress, and I am happy with my F/M ratios- always over 50%F.
Sorry I can't be more helpful- never tried 18/6.
stinkyattic Reviewed by stinkyattic on . vegging who has tried both th 18/6 and 24hr veg cycle?which best ,,what diferences are there? doesnt the 24hr stress the plants? can u use 24hr straight from seedling? Rating: 5