[nicoburns]

Maybe you're not actually deficient, even though in theory you could be. I used to get regular seasonal depression, to the point that I'd struggle to get out of bed at times. And I'd crave sunshine. Of ot was sunny. I'd obviously tried a lot of things. Changing diet, exercising, and also SAD lights which helped a tiny bit, but not really. Regular supplementation with vitamin D has made this almost entirely go away. I'm a completely different person.

So while you not everyone who is feeling tired or depressed will have a vitamin D deficiency, I'd say that it absolutely can cause those symtoms.