Photic sneeze reflex. Computer artwork representing one explanation for the photic sneeze reflex. About a quarter of the population sneeze when they look at a bright light, such as walking into bright sunlight. This mechanism is known as the photic sneeze reflex and serves no known biological purpose. There are various theories as to the cause. Sneezing occurs in response to irritation in the nasal cavity, which results in stimulation of the maxillary branch (centre) of the trigeminal nerve (yellow) and ultimately triggers a sneeze. One theory suggests that stimulation of the ophthalmic branch (upper centre) of the trigeminal nerve may enhance the irritability of the maxillary branch, resulting in an increased probability of sneezing.