The Photic Sneeze Reflex Affects About One Person in Four

In 1904, an author named L. Freund, who sneezed whenever he stepped from shade into brightness, tested the cause on himself. He exposed his eyes, face and nose to daylight, some of it filtered through coloured glass, and concluded that light reaching the eyes starts the sneeze, according to a 2025 review of the research. If you’ve ever walked out into the sun and sneezed before you could stop it, you’ve met what he was studying. It’s called the photic sneeze reflex (photic just means light-triggered), a sneeze set off by sudden bright light rather than by anything irritating your nose, and that same review puts it at roughly one person in four.

The light has to reach your eyes first

Freund’s tests pointed to the eyes as the main trigger. In his experiments, light filtered through red or green glass stopped or failed to bring on the sneeze, while violet and blue seemed to make it more likely. Light on the nose with his eyes covered could still do it on some occasions, so he never ruled the nose out completely.

Then the sneeze waits a few seconds

The sneeze doesn’t arrive the instant the light does. A 1985 study of English participants measured delays of 2.5 to 15 seconds, according to a 1987 paper from the Cleveland Clinic. Its author, neurologist Harold Morris, took that long delay as a sign that the sneeze isn’t a fast shortcut from the optic nerve to the brainstem. So where do those seconds go?

Your nose tickles, yet nothing measurable happens there

Most photic sneezers describe a prickle in the nose first. In 2009, Hydén and Arlinger put electrodes in the nasal cavity of seven people, three of them known photic sneezers, and flashed strobe or halogen light at their eyes. The sneezers said the tickle appeared instantly, but no matching electrical activity could be recorded in the nose from any participant, as the 2025 review reports.

The brain seems to assemble the tickle

A 2010 EEG study compared ten photic sneezers with ten people who don’t have the reflex. The sneezers’ visual cortex was more excitable, especially in a region called the cuneus. The stronger a person’s nose-prickle, the more active two areas that handle bodily sensation were, the insula and the secondary somatosensory cortex. The brightness of the flash didn’t predict it, and the brightest flash didn’t always produce the strongest tickle. The researchers concluded that this isn’t a classic brainstem or spinal reflex.

That points to the aha moment. The tickle you feel in your nose may be what the review calls a referred sensation, a feeling your brain places in the nose without any nose activity to match.

Three ideas for how the signals cross

Researchers have proposed three explanations, going back to a 1964 review by H. C. Everett:

• Optic-trigeminal summation. The optic nerve and the trigeminal nerve, which carries sensation from the face, cross-talk in the midbrain.
• Parasympathetic hypersensitivity. Photic sneezers have a generally more excitable parasympathetic system, the branch of the nervous system that runs tearing, pupil size and nasal secretions.
• Parasympathetic generalization. Strong light constricts the pupil and can trigger tearing, and the excitement spreads to neighbouring branches, including the one that makes you sneeze.

All three sound plausible, the 2025 review says, but no solid anatomical or functional evidence yet decides between them.

Why it runs in families

Morris described two families in which the reflex followed an autosomal dominant pattern (one copy of the trait is enough) with variable expressivity. Some relatives sneezed after eyebrow plucking or hair pulling instead of light. The group that gave it its acronym, ACHOO, in 1978 also called it benign. A 1993 report in Military Medicine examined it as a possible risk for combat pilots.

How common is it? That depends on who you ask:

The first seven rows come from Morris’s 1987 summary, and the groups, methods and years differ. A genome-wide scan of those 3,417 people in China found two regions of the genome associated with the reflex, according to a 2019 Scientific Reports paper. One was already known, and one was new, on chromosome 3 at a spot called 3p12.1. A similar scan of 11,409 people in Japan had also turned up variants at 3p12.1, so two populations point to the same region.

What nobody has measured yet

For something described for more than a century, the basics are thin. The 2025 review found only seven studies on the light itself, four case reports and three laboratory experiments, and none examined how the amount, colour or timing of light relates to sneezing. What kind of light triggers it isn’t pinned down, and none of the three ideas has been conclusively validated. The authors call for experiments that systematically vary the light’s wavelength, intensity and duration.