Btacker - light is needed for photosynthesis. With the lights out in the main tank, there is an increase in CO2 causing the PH to drop. If you have lights on in the fuge, photosynthesis will occur there and will help use up some of that CO2 being produced by the organisms in the main tank. Running the lights in a reverse cycle is a way to help keep levels somewhat balanced in the system. As others have mentioned, this is just an option, you will find people that use both methods as they both have their advantages. Hope that helps.