i would say that lower light periods would give you a decreased yeild for sure, but whether it works directly proportional like that, i am not sure if that is one hundred percent correct,, the only thing that you may have to worry about by interupting dark periods often is that it can cause stress and make your plants turn hermie.. if by interupting the dark period you mean turning the lights on for a half an hour or so everyday that is, but if your just talking about not 24 hours of light, don't worry about hermies, they will be fine..