In all fairness, this has been addressed before by one of my fellow vampire enthusiasts, David MacDowell Blue. A recent discussion on Twitter, however, inspired me to revisit the topic. Exactly where and when did the whole thing about vampires bursting into flame or crumbling into ashes in direct sunlight begin? Where did it come from? It would seem to have been, at least largely the creation of F.W. Murnau, who used it for NOSFERATU. But did he make it up whole cloth? That’s what I want to know.
Poring over the collected folklore, vampires weren’t bothered by sunlight back in the Dark Ages. (Almost a pun, there. Almost.) In DRACULA, Bram Stoker has his vampire Count walking around in the sunlight, but mentions that vampires aren’t as powerful in the sunlight. (Except at noon, when Dracula becomes stronger. Weird, right?) So was there already some tradition of vampires at least being weakened in the sunlight, or did Stoker just make that part up? This bears further investigation. (At this juncture, the only thing I can state with absolute certainty is that, whether they crumble into ashes or traipse around just fine in the sunshine, vampires do NOT sparkle.)
I expect it all has to do with humans’ instinctive fear of the dark. Also, the “encounters,” whatever they really were, that inspired and fed the fear of vampires tended to occur at night. Blame it on Night Terrors? Sleep Paralysis? Nightmares? Whatever the case, they happened at night, which evolved, I suspect, into the trope of vampires only being active at night, which led to the belief that they were afraid of the sunlight, or weakened or harmed by it. That’s my theory.