Can anyone give me some more specifics on Seasonal affective disorder?

I have looked it up but they give you the same symptoms. I was told i was low on vitamin D in november when i started getting depressed, so it would make sense if the lack of Vitamin D caused by lack of sunlight is making me feel like this. I have been taking the vitamin every morning since but i still have bad weeks and good weeks. Every morning i feel horrible, i usually don't get enough sleep since i'm a teen but i never used to feel this bad, the feeling lifts throughout the day and comes and goes throughout the day. I live in newyork so i don't get much sun, and i went tanning with my friend because it was free, i didn't plan on it and tought it was pretty dumb, but it made me feel better for a few days actually. And also am i more likely to suffer from this disorder if i go from having sun everyday in the spring summer and early fall, to barely any in late fall into winter?