Wikipedia: Photic Sneeze Reflex

Photic sneeze reflex is a medical condition by which people sneeze with sudden exposure to bright light, and possibly also to sneeze many times consecutively. It is also referred to as photic sneeze response, sun sneezing, photogenic sneezing, the photosternutatory reflex, or even whimsically as ACHOO syndrome with its related backronym Autosomal dominant Compelling Helio-Ophthalmic Outburst syndrome. The condition occurs in 17% to 35% of humans. The condition is passed along genetically as an autosomal dominant trait.

The probable cause is a congenital malfunction in nerve signals in the trigeminal nerve nuclei. The fifth cranial nerve, called the trigeminal nerve, is apparently responsible for sneezes. Research suggests that some people have an association between this nerve and the nerve that transmits visual impulses to the brain. Overstimulation of the optic nerve triggers the trigeminal nerve, and this causes the photic sneeze reflex. Another theory suggests that tears leaking into the nose through the nasolacrimal duct are a cause of the photic sneeze reflex. The speed of the reflex seems to favour the first theory, as it happens much too quickly for tears to be generated and drain into the nose. In addition this sneeze reflex can be brought on by a sudden inhaling of cold air or a strong flavour such as a strong mint gum. This implies an overstimulation of any nerve close to the trigeminal nerve can cause the sneeze reflex.

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I always wondered....