I agree with latewood.. extend the dark period to change your 12/12 cycle. If you have to change it turn it off at 6pm.. then don't start em up again till 7pm the following night.
I started my grow on 18/6 from 6pm till 12pm, now changed to 14/10 6pm till 8am, about to go to 12/12 6pm till 6am. Its summer here, I work all day and don't have time to piss fart around with them during the day and its cooler at night to run the lights.