uhhh...why would you do that . i assume that you mean during veg, but that would be giving them 12 hours of dark after 24 of light. i've heard that the maximum amount of darkness for vegging plants is 6 hours. i would think that would slow down your grow a lot, if it survived. i don't know what that would do for flowering, but i think that would be too much stress. isn't it the plant's nature to flower under equal parts of light to dark?