And, has anyone figured this out: Is it the light that falls on your skin that counts, or the light that falls on your retina?

If it's your skin, I would think you could benefit just by wearing skimpier clothes (indoors, I mean). Or using a tanning booth.

If it's your retina, I would think a few minutes of staring into a small bright light (if it's not too bright) would help. Maybe even turning up the brightness on your computer screen! Also, I would think a lot of blind people would be permanently depressed. Come to think of it, some (not all) blind people I have known did seem depressed!

It would depend on the kind of blindness, of course. Some blind people have their retinas intact, but their vision is clouded or out of focus.