Have you ever looked up into the sky on a bright sunny day, caught a glimpse of the sun, and sneezed? This sneezing phenomenon, AKA “sun sneezing” or the “Achoo Syndrome”, is called the photic sneeze reflex. According to MedicineNet.com, the photic sneeze reflex is a “disorder characterized by nearly uncontrollable paroxysms of sneezing provoked in a reflex fashion by the sudden exposure of a dark-adapted subject to intensely bright light, usually to brilliant sunlight.”
About 17 to 35 percent of the population is affected by this reflex. Typically when it happens, a person sneezes two or three times, but cases have been known to sneeze even up to 40 times.
Although there is no hard scientific evidence to explain this phenomenon, a hypothesis developed by psychiatrist Henry Everett in 1964 suggests that the reflex could be caused by nerve signals in the brain. According to the hypothesis, there are two reflexes which may play a role in sun sneezing.
The first reflex, called the pupillary light reflex, controls bright light which enters the eyes. When bright lights enter the eyes, this reflex sends signals to the brain through the optic nerve. This returns signals back to the eyes which allow them to constrict the pupils.
The second reflex is the sneeze reflex. This reflex identifies a tickle in the nose. The brain is alerted of the tickle, which results in a sneeze. For most people, these two separate reflexes (although very physically close to one another) do not cross. The hypothesis claims that for sun sneezers, these reflex signals cross paths when exposed to bright light. A signal is sent to the brain to constrict the pupils when exposed to the bright light, but when the signals crosses paths with the sneeze reflex, a sneeze results.
An alternative theory states that the medulla oblongata (the part of the brain which controls involuntary processes like sneezing) may cause this reflex. When the pupils are constricted from sunlight, the signals which flow to the brain could possibly get mixed up.
If you’ve ever known anyone who sneezes in the sunlight, or if it has ever happened to you, you may now have an answer!