While certainly not as dramatic as bursting into flames, for some people, sudden exposure to sunlight produces an unexpected reflex – they sneeze. Chances are this happens to you, or one of your friends. It’s called the ‘photic sneeze reflex’ and is more common than you’d expect, occurring in 17 to 35 percent of the world’s population, according to informal surveys. But what causes it?

Sneezing can’t really be controlled – it’s one of the body’s reflexes, and is typically associated with irritation in the nose. From here the signal is sent via neural pathways to the brain, resulting in a powerful release of air through your mouth and nose, which not only helps expel mucous or irritants from the nasal passages as fast as possible but also contracts a bunch of muscles in the body, including the eyelids and the trachea.

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When it comes to sun sneezing, even Greek philosopher Aristotle famously noticed the phenomenon and mentioned it in the ‘Nose’ chapter of his Book of Problems: “Why does the heat of the Sun provoke sneezing, and not the heat of the fire?”

However, the photic sneeze reflex has nothing to do with heat and instead appears to be the result of crossed wires somewhere along the trigeminal nerve. Also known as the fifth cranial nerve, it’s the largest and most complex paired nerve in the head, with three major branches leading to the eyes, nasal cavity, and the jaw. It’s a crowded place in terms of nervous signalling, so it’s not surprising that the trigeminal nerve would occasionally get the reflexes wrong. Bright light causes your pupils to contract, so that signal might be mistakenly sent to the nose as well.

source sciencealert.com