"Challenging" isn't the word. Try "miserable."

I was under the impression that once the sun is below about a 45° angle in the sky - basically much of late fall/winter in the North - you can't produce a useful amount of vitamin D. People I know who ski can get a burn, but that takes all day. A rule of thumb I heard (probably here somewhere) is that once your shadow (or the shadow of a tall thing like a lamp post) is longer than it is tall, there won't be much vitamin D production.

I can easily see how people today don't get enough sunlight in winter. Do they not have tall buildings in Boston? Toronto, for example, is a wind tunnel. A cold-baby like me living in downtown TO would probably go straight from their condo to the subway station connected to the building, walk through the Path to work, do their shopping there after work, go back the same way, and rarely even go outdoors. Suburbs are just as bad: kids in my neighborhood get chauffeured to school and back, even though the school is almost immediately behind our townhouse complex. It's probably worse in the US where walking anywhere is dangerous.

Throw the SAD on top of that, and you're in trouble.

I think they're underestimating how many people can fall into this category.