anecdotally my doctor recently recommended I take a shitload of vitamin D because I'm a troglodyte, and I do think I feel a little better lately

this was while it was still summer too, and I have whatever the opposite of SAD is so it wasn't just the onset of fall making me feel better

I believe the link between vitamin d levels and seasonal affective disorder is confirmed, but afaik doctors usually prescribe light therapy with a sun lamp to stimulate bodily vit. D production, not supplements. I've seen studies saying supplements seem to work, and studies where they had no effect, so its inconclusive but worth a shot.