During vegetation my photoperiod is 18/6 not unless I have autoflowering strains in vegetation. Autoflowering strains I go 24/0. Prior to flowering I place them in complete darkness for 3 days to get them aclimated to the photoperiod of 12/12.
If its a auto flowering strain just leave it on 18/6 why switch it to 12 /12 all your doing is losing out on yield auto's will automatically flower even on 24 hr lighting
Keeping plant in darkness 3 days prior to 12 /12 is a waste of time and los of 3 days growth it only induces stress, where on earth other then the poles do you get 24 hrs of darkeness ( POLES ) and nothing grows there keep it simple keep it closest to mother nature
Think of a plant like your self you work all day then go to sleep a plant doesn't sleep it works 24 hrs a day it will convert light to energy on lights on and store it lights out it will place it all over the plant from roots growth to overall growth lights off are needed
24 hr lighting creates very tight noding shorter plants
18/ 6 is pretty much closest to mother nature in veg and by far the best schedule for any plant it produces the best overal growth through out veg
12 /12 again is like mother nature producing autumnal equinox converting a vegging plant to flower mode