The link between Vitamin D deficiency and depression is not really new, but now that the Mayo Clinic is linking the two, perhaps it will be taken a little more seriously. Check out this article from WebMD. Also, please note that the RDA (recommended daily allowance) for Vitamin D listed in the article is below what many doctors are now recommending (I take 5000 IUs daily and in in the winter, I alternate days of 5000 and 10,000 IUs just to keep my levels optimized).
I used to live in the Northeast, where in the winter we wouldn't see the sun for a week or more at a time. Seasonal Affective Disorder (SAD) is very common in the winter time, short days, people aren't outdoors to even feel the effect of the sun on the spirit (we are on day three of gray, gloomy weather here in NC, I just want to stay inside and knit!) Even when the sun is out in the winter, if you live in the northern two thirds of the country, the sun is not strong enough to benefit your vitamin D levels from about mid October to late March. Be sure to check with your doctor, and if you have not had a Vitamin D test, insist on one at your next visit. Almost every person I know who has had the test found they were deficient in Vitamin D.
Off to knit (no, just kidding, I am in the house for the rest of the day, but I have many little things I need to catch up on around here!)-----sunny or cloudy, enjoy the rest of your day!