Morning all.

I need to change my light cycle so they come on at night (make use of economy 7) but at the moment my plants are on 12/12 from 7am - 7pm. Do I just bring it back by an hour a day or can I do more than an hour? My plants are in their 2nd week of flowering so I really don't want to stress them out (fem seeds). Any advice would be greatly appreciated.