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.”
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.