I think there's more to SAD than just the lack of sunlight, Helpinghand. Winter is limiting. It's cold out, so you have to bundle up to go out, and you can't do the things you can in the springtime or the summer. There's snow, or sleet, or ice to deal with. Sometimes, it's just too darned miserable to set foot outside. Those kinds of limits can really tend to bring people down; even those who don't suffer from depression. We're stuck in the house with little to do and we get the heebie-jeebies. I really think that's a good part of it.
1. Even on a sunny winter day, the number of hours of daylight is reduced. For example, at the moment in Ontario we have daylight from about 7.20 am to 4.20 pm (about 9 hours) as we move toward the winter solstice (shortest day of the year) in about 3 weeks. By late March, we'll hit the equinox (12 hours daylight, 12 hours darkness) and by late June we'll hit the summer solstice with about 15 hours of sunlight.
2. Some people report experiencing a summer SAD as well as or instead of a winter SAD, although that's less common, so it seems it's not purely about hours of sunlight (I'm not certain this is truly SAD, mind you).