One in three people sneeze when exposed to bright light. But why? People have wondered since the 3rd century BC!
Sneezing due to exposure to bright light, like sunlight, is known as the photic sneeze reflex. About one out of three people are born with this genetic quirk which scientists are still unable to fully explain.
Mostly it is unexplained because there has not been that much research done about it yet. There have been attempts at explaining it, and even Aristotle wondered and wrote about it in his 'Book of Problems' in the 3rd century BC. He proposed that the heat of the sun was probably responsible.
A very simple demonstration by Francis Baker, some 2000 years later, proved Aristotle wrong. He stepped into the sun with his eyes closed and did not sneeze, despite the presence of the heat. He then surmised that the light makes the eyes water, which in turn irritated the nose, resulting in a sneeze.
Neurological experts now agree, however, that the phenomenon is a result of crossed wires in the brain. When we are suddenly exposed to bright light, the optical nerve typically sends a signal to the brain to constrict the pupils. In people with a photic sneeze reflex, some of that signal is picked up by the trigeminal nerve. That is the nerve that is triggered by an irritation in the nose. Due to this mixed message received by the brain, it registers the signal as an irritation in the nose, resulting in a uncalled-for sneeze.
Although the gene responsible for this 'sun sneezing' has not been identified yet, a publication in 1978 was more concerned about renaming the photic sneeze reflex. They suggested Autosomal-dominant Compelling Helio-Ophthalmic Outburst syndrome: ACHOO.
Well, bless you!