According to Feldman, there's some evidence that beta-endorphins can't actually cross the blood-brain barrier, meaning that they would exert most of their influence elsewhere in the body, especially on the skin. But he wonders whether in some people endorphins can and do cross into the brain—and whether these are the people for whom UV is more likely to be addictive. Feldman doesn't have evidence for this, but it's an interesting bit of speculation.