Coraline likes low light, but for how many hours is another thing. It is said to atleast run your photoperiod for 8 hours a day. Why is it that you want to run a shorter photo period? Anything to do with nuicance algae growing in your tank with the longer photoperiod? If so, you can probably run a blue spectrum (blue actinics) for a few hours, because from what I have learned, coraline loves the blue spectrum (algae not so much)and then run a 50/50 spectrum for the rest of the day. Just a thought...Not really sure why you wanted to run the shorter photoperiod...