Great article, thank you for sharing. I live in Alaska, in the interior where we get about 4 hours of daylight around Winter Solstice on December 21. I take 5000 IU of D3 a day, in combination with 100mcg of K2, which I have read helps with utilization. It's difficult to get out and soak up the sun when it sets at 3:30PM. I have not felt the winter blues, so I have to say that D3 by itself seems to work. It's the safflower oil source, the label says is the most bio-available. So from personal experience, I have to say that form of D3 is adequate to keep me from getting the winter blues!