From the research I've done it seems best to put them in a 24-36hr dark period before turning the lights on for flowering. It makes them show their sex in as early as 2 days into 12/12. Supposably the dark makes them think their dieing and they create pollen or flower depending on their sex, and its suppose to make the transition into flowering easier on them. Also, when the nodes are alternating instead of non alternating it shows the plant is maturing.