Why are vampires scared of sunlight?

Answer 1

It has to do with ultraviolet rays and a vampire's melanocytes. UV rays are a form of ionizing radiation, which does heavy damage to the DNA structure of cells. Because ultra violet rays are at such a short wavelength (anywhere from 400 nanometres to 10 nanometres), they are especially dangerous for the cells that undergo melanogenesis. In a normal human, this damage causes your melanocytes to release more melanin and darken the pigment of the skin, ie. get a tan. In vampires, melanocytes are underfunctioning, and the photodamage to the melanocytes causes such a drastic shift in melanogenesis that cells' lysosomes actually initiate apoptosis. Dead cell matter at such a high concentration puts so much pressure on surrounding tissue that the blood vessels can become blocked, resulting in pressure buildup and the eventual failure of the cardiovascular system. Normally, the process takes somewhere from 10 to 15 minutes, and the vampire looks as though he or she died of heart failure. In extreme cases, the sudden blockage can result in internal bleeding and rhabdomyolysis from sudden internal shock on surrounding muscles. In this case, death can be instaneous of contact with sunlight.

Answer 2

In traditional vampire lore, sunlight is believed to be harmful to vampires because it represents purity and goodness, contrasting their dark and malevolent nature. Exposure to sunlight is said to weaken or even destroy them, emphasizing their supernatural vulnerability to forces of light and life.

Answer 3

That is what the authors of the stories decided. It could have been anything. It has now been perpetuated by the authors of new stories.