You can have a light on when your in there, 15-20 minutes of light isn't gonna hurt then.
I don't mean HID light. I'm talking about regular incandescent light. up on the ceiling.
IMO- I'd treat it like foliar spraying. you want to spray in the first 15-20 minutes from when the HID's come on. (don't spray your bulbs)
If your spraying in the dark, you run the chance the water won't evaporate, your buds get mold, if you don;t have the air movement when it's dark.
If the stomata are not open. all your doing is washing the leaves off in the dark cycle.