You are going from 24 hours of light to 12. I hope you have a timer. An important part of the dark cycle is that it stays dark. No light leaks anywhere. I used towels and tape to make my first room totally dark. So go in there and check that is completely dark. Then if your going to start dark at say 6m...just change your lights before then when they come on in 12 hours it will be the HPS... make sense?
I like to run my lights at night to help control the heat. So, right now, my hps comes on at 1 in the morning and goes off at 1 in the afternoon. This gives me plenty of time to care for the plants in the mornings and the lights are turned off during the hottest part of the day.