Excellent question. The Sun's rays are a form of non-ionizing radiation, essentially just RF at extremely high frequencies. Of course, we evolved for billions of years underneath the Sun, so there's that. For myself, I think moderate exposure is best. Dark tans, or burns, are a sign that you've gone as far as you ought to go, or too far. We Europeans evolved at relatively high latitudes (Venice is further north than Minneapolis, for one example), where the rays are less intense than average, and I think we spent a lot of evolutionary time in shady forests. Some sunlight every day is a good thing for your health, but browning yourself in the planetary microwave oven seems like a bad idea.