To what extent is my light cycle likely to affect my yield?

I'm currently growing an AutoFresian Dew by Dutchpassion alongside a photoperiod plant. I want my photo done for Xmas so have switch the lighting in my grow space to 12/12. Is this going to make a big difference to my yeild? Like, will 50% less light mean 50% less yeild? I know fresian dew was bred to grow outside in a northern latitude so maybe its not so bad?