Dark Room Questions

Recently ive switched from 24/0 to 20/4. (the girls seem to love their nap time)
Im not using a timer for my lights due to the fact that i have different aged plants/seedlings in my grow room.
My question though, is a dark room ok for my plants to transfer to when they go for their sleeping time?
The dark room is a completely sealed off, clean closet where no light can leak through. Also should i put a fan directly on the plants while their sleeping? I already have an exhaust fan for fresh air to get in.
Please any response would help
thanks all.

what's the problem with just getting them all onto 20/4? or are some actually in flowering as well?

but yes, if you have a nice clean dark room with ventilation and a circulation fan, and temperatures and humidity is good, then that'll work just dandy. sounds like a hell of a lot of work though i must say