First, autoflowering strains don't care about light/dark period, in fact you shouldn't grow them under 12/12, you should do either 20/4 or 24/0. They can handle all the light you can throw at 'em, so give it to 'em. Anything less and you're reducing your yield.
Second, non-autoflowering strains can hermie from light pollution during "lights off" but not from extended darkness periods. What this means is you cannot interrupt the 12 hours of darkness, you can turn the lights on and off all you want during the "lights on" period, you can leave 'em in complete darkness for 24 hours, except for being hungry the plants won't care, but disturb the darkness period and you may get hermies.