This is not even remotely surprising to me. I've always been a bit skeptical of "the sun is evil" (and got even more skeptical when I was told I should wear sunblock inside!) and ensure I get 20 minutes full body exposure daily in the summer. Anecdotally, I can tell a general difference in my mood and sleep during periods where I get adequate sun vs. when I'm not (and I supplement vitamin D when I'm not).

While I am a big fan of the scientific method, it is difficult to do studies that don't have lots of confounding variables in this area, so when in doubt, use common sense is my rule. We evolved with the sun, so the most likely probability is we have at least some defense to it, and more than likely we evolved so that at least some exposure has benefit.