Myth or fact? Who knows for sure, but it's total BS in my opinion, unless someone can show me some real proof or evidence for once. Plants need a couple things to thrive, and one of them is light. In my humble opinion that's just 3 days that you are depriving it of something the plant needs to help it grow. And even if it happens to be beneficial, does it outweigh the benefit of three days of proper light?