Photic sneeze reflex

How it works

A normal sneeze starts when something irritates the lining of the nose. The trigeminal nerve carries that signal to the brainstem, which fires the sneeze³. In a photic sneezer, a sudden bright light does the same thing. Stepping out of a dark cinema into sun is the classic trigger. Looking at a bright lamp or a camera flash works for some people too¹.

The cause is not fully proven. The leading idea is cross-talk between nerves. The optic nerve, which carries light signals from the eye, runs close to the trigeminal nerve, which serves the face and nose². A strong burst of light may leak signal into the wrong pathway. The brainstem reads it as an itch in the nose and sneezes. A related idea is that the nervous system of photic sneezers is simply more excitable, so a big jump in one sense spills over into another¹.

Most people sneeze two or three times per burst. Up to 43 sneezes in a row have been reported¹. Sunglasses or a hat reduce the effect¹.

The reflex is inherited. It is autosomal dominant, which means a child of one affected parent has about a 50% chance of having it¹. A 2010 study using 23andMe data found gene variants linked to it¹.

How common it is

Surveys put the reflex at 18% to 35% of people, and about one in four is the usual figure¹. Many people who have it assume everyone does.

History and culture

Aristotle asked why the sun makes people sneeze in the fourth century BC¹. The acronym ACHOO, for autosomal dominant compelling helio-ophthalmic outburst, was coined as a joke and stuck¹. The reflex is a real concern for pilots and drivers, because a sneeze closes the eyes for a moment at the worst possible time, such as coming out of a tunnel into sun¹.