Why do some people sneeze when they look at bright light?

The photic sneeze reflex, sometimes called ACHOO syndrome (Autosomal Dominant Compulsive Helio-Ophthalmic Outbursts), causes people to sneeze when suddenly exposed to bright light, especially sunlight. The exact mechanism is debated, but the leading theory is that the trigeminal nerve, which triggers sneezes, runs close to the optic nerve. A strong light signal may spill over and stimulate the trigeminal nerve. The trait is inherited in an autosomal dominant pattern, meaning a child has about a 50 percent chance of having it if one parent does.