Scientific American reports that scientists still do not know for sure why people sneeze when exposed to bright sunlight. According to NBC News, there are two conflicting theories about why people might sneeze in response to sunlight.
NBC News reports that one theory for why the sun makes people sneeze is that the visual system could be over stimulated by the bright light, which activates regions in the brain that control sneezing. Another possibility is that the nerve that carries signals from the eye to the brain stimulates the nerve that connects the brain to the nose, causing sneezing. According to Scientific American, photic sneezing runs in families and probably has a genetic cause.