Given that SAD is so tied to the seasons and to sunlight in particular, you might predict that people who live in places with cold, dark winters would be more likely than people living in sunny climes to experience SAD. You'd be right.
For example, the rate of SAD in New Hampshire has been estimated at 10%, whereas less than 2% of people living in Florida suffer with the condition. In fact, I've known many patients over the years who only developed SAD when they left places like Florida to live further north.
In this way, SAD is the quintessential environmental disorder -- no winter, no SAD. But that is not the whole story.
Like all psychiatric conditions, SAD occurs at the intersection of genes and environment, of nature and nurture. This point has been brought home by several remarkable studies showing that people in Iceland have remarkably low rates of SAD, despite living in one of the darkest winter environments on earth.
Even more remarkably, people of Icelandic descent living in the prairie provinces of Canada have far lower rates of SAD than their fellow non-Icelandic Canadians, which greatly strengthens the argument that Icelandic people carry an as-yet-undiscovered genetic factor that protects against SAD.
We don't know what causes SAD, although abnormalities in multiple brain regions have been repeatedly observed in studies. The neurotransmitter serotonin seems to be implicated, as does melatonin, one of the hormones most involved in the onset and offset of sleep. Perhaps most intriguingly, several studies suggest that the eyes themselves might contribute to the risk of developing SAD, given that the eyes of people with SAD respond differently to light.
There also seems to be a strong association between being affected by the seasons and having heightened creative abilities.
I was a practicing psychiatrist before I fully admitted to myself that I suffered from mild SAD. In fact, I didn't really believe it until I bought my first light box and turned it on one fall.
The results were amazing. Thirty minutes in front of a bright light (10,000 lux to be exact) and my brain and body felt like they'd been transported back to summer. My mind knew it wasn't true, but that didn't matter; the melancholy was gone.
Of course, my experience is hardly unique, and if you suffer from winter depression you really owe it to yourself to buy a light box and give it a try. You might also look into dawn simulators, which have also been shown to treat SAD.