I hadn't heard of SAD before now but I have suspected for some time that my partner suffered from depression brought on by seasonal change.
He becomes terrible depressed through the winter months. I can see/feel it building like this big black cloud through late April, May and by June it's full blown - he'll literally curl up in bed and turn his face to the wall for days if I let him when he's really bad.
Oh...I'm in Australia so the seasons are reversed (obviously lol)
Perhaps I should look into this light thing and suggest it to him. Anything to help! Thankfully Spring is now well and truly on the way and he's on the up. Yay~!