I saw this TED talk a while back about how sunshine releases big stores of Nitric Oxide stored in your skin. Apparently this has positive health effects like for heart health. But regarding pain, Nitrous Oxide is also basically what laughing gas is, the popular dentistry pain killer, so perhaps elevated levels in the blood are doing something to suppress nerve pain? Could explain the different effect of sunshine vs just vitamin d supplements, you don't get that big nitric oxide release with just the D supplements.