The short answer

The photic sneeze reflex is likely caused by wires getting crossed in the face's nerves. A signal sent from the optic nerve when there's sudden bright light might be inadvertently triggering the nose to sneeze.

The long answer

For most, the idea that bright light can trigger a sneeze seems peculiar.

But for the remaining 18-35% of the population, a photic sneeze reflex is a way of life. This unusual condition is also known by its official (and officially cute) name of autosomal dominant compelling helio-ophthalmic outburst syndrome – or ACHOO syndrome.

ACHOO syndrome seems to range in severity. I personally take advantage of this condition as a hack to get a sneeze out by staring a bright light when I feel a sneeze building up. But for others it can cause an uncontrollable sneeze or sneezing fit every time they leave a dark room. ACHOO syndrome can be downright dangerous in situations like driving a car out of a dark tunnel when sneezing can take your eyes off the road.

In fact, there was a study back in the 1990s focused on combat pilots to see if any measures (like specially designed sunglasses) could prevent the onset of an outburst. Unfortunately, they found that the condition was not triggered by "specific wavelengths of light, but rather by the change in light intensity."

The causes are not well-studied, but the best theory about why some people have a photic sneeze reflex has to do with the trigeminal nerve.

The trigeminal nerve is responsible for most of your face's feeling and, as the name implies, connects to three parts of your face. Two of those parts have to do with your nose and your eyes.

The theory goes that a sudden change in light activates the eye-branch of the trigeminal nerve in order to constrict your eye's pupils. This signal may be crossing over into the nose-branch of the trigeminal nerve, stimulating it to sneeze. In other words, our wires get crossed.

Check out some other curious questions:

Sources

Breitenbach, R. A., Swisher, P. K., Kim, M. K., & Patel, B. S. (1993, December). The photic sneeze reflex as a risk factor to combat pilots. Military Medicine. https://pubmed.ncbi.nlm.nih.gov/8108024/

Goldman, J. G. (2015, June 24). Why looking at the light makes us sneeze. BBC Future. https://www.bbc.com/future/article/20150623-why-looking-at-the-light-makes-us-sneeze

Griffin, J. (2017, March 20). Why looking at the sun can make you sneeze. PBS NewsHour. https://www.pbs.org/newshour/science/does-the-sun-make-you-sneeze

Higuera, V., & Biggers, A. (2019, June 10). Why Do I Sneeze in Bright Light (and Other Unusual Stimuli)?. Healthline. https://www.healthline.com/health/photic-sneeze-reflex

Massachusetts Eye and Ear. (2018, November 28). Why Does the Sun Make Some People Sneeze?. FOCUS. https://focus.masseyeandear.org/why-does-the-sun-make-some-people-sneeze/

Veritasium. (2017). The Sun Sneeze Gene. YouTube. Retrieved September 14, 2023, from https://www.youtube.com/watch?v=e69XZJ9DEj0.