Sun Therapy

I used to practice this theory when working at the nursing home, I called it "sun therapy". However, this article from the NY Times agrees, this works even on an overcast day.

They indicate that the ultraviolet wavelengths of 290 to 320 nanometers (in the ultraviolet-B range) are necessary for an adequate "sun bath".

I love a good "sun bath", however they also caution the ultraviolet-B waves are also associated with sunburn and skin damage. So, just like in most other cases, use in moderation.