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Photic sneeze reflex

How can looking at sunlight make you sneeze?

Discover magazine: Sneezing provoked by sudden exposure to intensely bright light is known as the photic sneeze reflex. It is not uncommon--about one in ten people are photic sneezers. Some studies suggest it may be due to an accidental crossing of nerve signals involved in normal sneezing and pupil dilation. But the photic sneeze reflex occurs only after someone has been adapted to the dark for at least five minutes. Even photic sneezers' pupils will adjust normally (without triggering a sneeze) if they go in and out of bright light for short periods of time, so the real mystery is why an extended period of darkness makes a difference. The reflex is not troublesome for most people (a notable exception are fighter pilots), but it is of considerable interest to scientists. Studying the photic sneeze reflex is likely to improve our understanding of sensory pathways in the nervous system, as well as the circuitry of the sneezing reflex.