If you find that whenever you walk out into bright sunlight, you sneeze, you may be part of around 25 per cent of the population who have what’s called the photic sneeze reflex (PSR).
It’s not an allergy to the sun but a reflex response. Despite being documented for over a century, scientists are still scratching their heads over why it happens.
What is the photic sneeze reflex (PSR)?
The PSR is an involuntary reaction triggered by exposure to bright light – especially sunlight. It’s often hereditary and has even earned the nickname “ACHOO syndrome” (short for Autosomal Dominant Compelling Helio-Ophthalmic Outburst).
But what causes it?
A recent review in Experimental Brain Research (2025) pulled together over a hundred years of findings and modern lab studies, and the results are inconclusive. Several theories try to explain the PSR:
- Optic-Trigeminal Summation Theory: Bright light stimulates the optic nerve, which may “cross-talk” with the nearby trigeminal nerve (which controls sneezing), like a neurological short-circuit.
- Parasympathetic Hypersensitivity: Some people may have a generally oversensitive parasympathetic nervous system.
- Generalised Parasympathetic Response: Bright light triggers other related reflexes (like pupil constriction), which could spill over into sneezing.
Despite sounding plausible, none of these have been conclusively proven.
Generally, having PSR poses little risk. However, if you’re driving or flying a plane and suddenly find yourself sneezing uncontrollably, the consequences could be serious. This is why military researchers have studied the reflex in pilots.
Understanding the PSR could help in broader research into reflexes, sensory disorders, and even conditions like photophobia (light sensitivity). But right now, scientists say more controlled studies are needed. Parameters like light intensity, colour spectrum, duration, and angle still haven’t been tested systematically.
Have you ever sneezed when stepping out into the sun? Does it run in your family? Let us know in the comments below.
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