Why does bright sunlight make people sneeze?
Sunlight sneezing is thought to be caused by a change in light intensity rather than by a specific type of light [1]. When your eyes are exposed to bright light, the parasympathetic nervous system, or the rest and digest response, causes the pupils to constrict to protect the eyes from light damage [2,3]. This effect may indirectly activate other parts of your rest and digest response, including those that control mucus secretion and the sneeze response in your nose [2].
Sunlight has been known to cause sneezing. This is because the sun triggers the photic sneeze reflex (PSR) or the ACHOO (autosomal dominant compulsive helio-ophthalmic outbursts of sneezing) syndrome [2]. In recent studies, it’s been shown that the reflex seems to be caused by a change in light intensity rather than by a specific type of light [1]. This is why bright lights, camera flashes and even the brightness from snow can sometimes cause a sneezing sensation [1]. It all starts with crossed wires involving the trigeminal nerve, a facial nerve that innervates the eye and nose [3]. When a bright light hits the eyes, the pupils constrict and this causes the nerve to send a signal to the brain to trigger a sneeze [3].