When a nerve cell is stimulated it passes on a chemical / electrical message to the next nerve cell in the chain. However, if it is a very strong message, this might also leak out and stimulate nearby nerve cells. So, when you look at the sun on a bright light, your eyes (and their nerves) suddenly have a lot of very strong information to pass to the brain. In addition to passing on their message, they also 'leak' a bit. Part of the path for the optic nerves (from your eyes) happens to be close to your sneeze reflex and so it can be triggered by accident.
-This text was originally contributed by David Hone on Ask a Biologist
Anecdotal evidence also seems to indicate that this unusual reflex has a genetic link, so may be common among members of the same family. This is confirmed by R. Eccles of the Common Cold and Nasal Research Group who says that this phenomenon (which he terms the ‘photic sneeze’) affects between 18-35% of the population. And that the photic sneeze is a well known hazard to fighter pilots when they turn towards the sun or are exposed to flares from anti-aircraft fire.
Sneezing In The Sun
After eight years and I am still learning new things my husband: we were walking down the street yesterday when the clouds parted, revealing a bright sun and he consequently sneezed! Now I realize that this phenomenon affects a significant number of other people I know. So in case you were ever wondering why looking at the sun on a bright day makes you sneeze here is the answer: