Aristotle noticed it. Around 350 BC he wondered why the sun makes people sneeze, and landed on a guess that sounds reasonable: the sun’s heat warms the nose, and the sneeze follows. It was a decent answer for the fourth century BC, and wrong.
Nearly two thousand years later Francis Bacon took a more careful path and closed his eyes. Facing the sun with his lids shut, the heat still reached his face, but the sneeze did not come. That small experiment moved the trigger from warmth to light, where it has stayed ever since.
What Bacon could not do, and what nobody has managed since, is explain how light hitting the eye ends up firing a reflex in the nose.
The thing has a name now, the photic sneeze reflex, and a fittingly overworked acronym, ACHOO, for autosomal dominant compelling helio-ophthalmic outburst. It was first reported in the medical literature in 1954, though the curiosity runs back through Bacon to Aristotle. Decades of study later, nobody can say for sure how it works.
How common it is, and why the numbers won’t sit still
The reflex affects 18% to 35% of people. That’s a wide band already, and the individual studies scatter even further. A study of 3,417 people in China put it at 25.6 percent, with men reporting it more than women. At the other end, a German study of 1,042 hospital patients found 57 percent, well above the usual range.
The spread may say as much about measurement as prevalence. These numbers are generally based on people reporting whether bright light makes them sneeze, and there is no standardized diagnostic test. Some people may never have noticed it; others may interpret an occasional sneeze differently. The 2019 Chinese study itself noted that attempts to reproduce the reflex clinically had been unreliable, making self-report the practical way to identify it.
There is a genetic thread running under all of it, but the inheritance is not as simple as the acronym suggests. Older family studies described the reflex as likely autosomal dominant, but that evidence came from very small pedigrees and was not conclusive. The Chinese genome-wide study found several associated genetic variants, including rs10427255 and rs1032507, and described the reflex as a complex rather than straightforward Mendelian trait. That is progress on the “who” of the reflex. It still does not answer the “how.”
Three theories, none of them nailed down
The core problem is a wiring problem. Sneezing normally begins with sensory input carried through branches of the trigeminal nerve. Light, meanwhile, enters through the visual system. So the puzzle is why a sudden burst of brightness should ever reach the circuitry that produces a sneeze.
A 2025 mini-review of the research lays out three leading explanations and says plainly that none has been conclusively validated. The first is optic-trigeminal summation: signals associated with bright light somehow cross into trigeminal pathways, producing the familiar prickling sensation in the nose and, in some people, a sneeze.
The other two theories put the crossover elsewhere. Parasympathetic hypersensitivity proposes that some people have a generally more excitable parasympathetic system. Parasympathetic generalization proposes that a strong light-driven response such as pupil constriction spills into nearby autonomic responses, including lacrimation and nasal activity.
The review’s conclusion is unsatisfying but clear: there is not enough anatomical or functional evidence to choose between them.
Why a harmless quirk is worth the trouble
A sneeze is trivial until the timing isn’t. A driver coming out of a tunnel into glare or a pilot exposed to a sudden change in brightness can have a moment where an involuntary sneeze matters. That is one reason the reflex has attracted attention despite being harmless for most people.
The bigger obstacle is practical. To understand the mechanism, researchers need to reproduce the reflex reliably in the lab, and the 2025 review found that the light conditions that trigger it are still poorly defined. Intensity, wavelength, duration, timing and spatial configuration have all been studied inconsistently, and no study had established a clear relationship between those features and the sneeze response.
That leaves an oddly stable mystery. We know the reflex is common, that it runs in families, and that bright light rather than heat is the trigger. What happens in the fraction of a second between light hitting the eye and the sneeze arriving is still the part nobody has caught in the act.