Vitamin D deficiency

Saw an article on TV that there wasn't enough sunlight in this country to produce vitamin D between Sept and April and that people who's bodies were incapable of storing it or who's diet was deficient in Vit D suffered symptoms very similar to SAD. I have started taking a Vit D suppliment and although early days I am feeling much better. Still use light on dark miserable days as a boost. But Vit D together with regular excercise (see article or sport and fitness) seems to work for me. Is Vit D defiency SAD ??