I heard this first-hand from a therapist though I didn't know if I really believed it at first. He said there were these breakthrough studies done that showed light- even artificial light (as opposed to natural daylight)- made a HUGE difference in helping rid depression, not just a little one like common sense would dictate. Also I notice my mom emphasizing opening the blinds more than usual in the day and it isn't till now I realize why she does this. I eventually noticed the difference light made in my depression as well. So light both artificial and natural really do impact how depressed you are or aren't.
So if your feeling down, get some light!
(And laughter. Studies done have shown laughter is good for you too).