I have found a strong correlation between what I eat and how much my skin reacts to the sun. I've been out as long as 2 hours in the Texas summer (zero sunscreen) without any kind of ill effect afterward. This is the best case though. If my diet has recently been trash, I can barely go 20 minutes before I start to feel it. I think consuming high concentrations of seed oil is a big part of what causes it. I don't know the exact mechanics of it biologically, but I do know that if I eat that whole bag of Sun Chips I'll effectively become a vampire for the next 3 days.