Interesting question, though I'm not qualified to answer on SAD. I use to work in London and literally not see daylight all work week during the winter, and it didn't bother me then, 30 years ago, too young to know better. However now, I do feel healthier when we have clear sunny days to walk outside over dreary ones. Can't imagine living again in New England where I was raised, and dont even like visiting northern cities in the winter..the earlier darkness bothers me now.

Also read all those retirement guides which rank cities on parks and recreation facilities. Bike-friendly?, greenway and trails?, indoor pools? golf? whatever you can foresee yourself doing to keep active year round. Are there Farmer's Markets and access to fresh foods? Good Medical care? Health and well-being is not just the sunshine. I've read many of these types of guides, and enjoyed the unique focus of What Color is my Parachute for Retirement? Yes, the author has grown old with us and is applying his skills to a new area
If you find a good place, let us know. We keep looking, but more sun and warmth hasn't trumped these other attributes yet