if you're going to do it, i'd try and do it as soon as the lights went out, so they still get pretty much all of their dark period..
like i had to go up and water my plants, but the lights had just gone out, but i wasn't too worried being it had only been about 5 or 10 minutes ago...
Yeah I agree with racerboy. Either right when they shut off or right when it turns on. In the middle of darkness is meant to be left alone and stay dark. Light stress can cause hermies, which starts seeds and and pollen issues.