Why does looking straight into a light or near the sun help you sneeze?

Question

Why does looking straight into a light or near the sun help you sneeze?

Answer 1

"A sneeze is usually triggered by an irritation in the nose, which is sensed by the trigeminal nerve, a cranial nerve responsible for facial sensation and motor control. This nerve is in close proximity to the optic nerve, which senses, for example, a sudden flood of light entering the retina. As the optic nerve fires to signal the brain to constrict the pupils, the theory goes, some of the electrical signal is sensed by the trigeminal nerve and mistaken by the brain as an irritant in the nose. Hence, a sneeze."

To read the whole article from Scientific American, check out:  http://www.sciam.com/article.cfm?id=look...

Answer 2

This is called the "Photic Sneeze Reflex", or, jokingly, ACHOO syndrome (Autosomal dominant Compelling Helio-Ophthalmic Outburst syndrome).

It is a genetic trait - an autosomal dominant condition affecting about 17-35% of the human population.

Quoting from wikipedia:

"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."