This form of depression is characterised by its routinized episodes. Generally episodes begin in late fall- early winter and last until spring. Thought to be caused by the lack of sun exposure (and therefore the lack of regulation of melatonin) it can be treated by phototherapy. The bright light mimics the suns rays and helps to restore balance.
SAD is particularly relevant for Canada as we have a large population living in the northern part of our country. I do not know the rates for countries in Northern Europe or Asia but I know that rates are higher in Canada than most (of course I cant remember now where I read that!)
My friend had SAD. She has a light box that she uses and she will sometimes go to the tanning salon (which was recommended to her by her doctor!) to get those rays she needs. I have always been jealous and bitter about those with SAD... it seems to me that they have such an 'easy' solution. But of course it is not as simple as that! I hear the light bulbs for those things are really expensive, plus you need the time to actually sit down and be around that light for a significant amount of time. Time and money plays a factor here. as with any treatment time, money, availability are huge factors in recovery. (more on that later)