Well, I know from an evolutionary perspective (thanks to the wonder that is Bill Nye the Science Guy) that the reason humans lost pigmentation in the skin as they moved into latitudes away from the equator is that vitamin D is produced when the skin is exposed to sunlight, not the eyes.

And I heard (relatively recently -- don't remember exactly when) of another study where it was concluded that it's the body's perception of sunlight hitting the back and shoulders that is most effective in fighting depression (and included the commentary about what are we doing to our kids, when we're making them sit inside all day, and even P.E. is indoors, and no wonder kids are suffering from more depression... etc. etc.)