Photic sneeze reflex
Also called: ACHOO syndrome · Sun sneezing · Autosomal dominant compelling helio-ophthalmic outburst
About one person in four sneezes when they step into bright sunlight. It runs in families, and the leading idea is a wiring mix-up between the nerve for the eyes and the nerve for the nose.
How common
Does this happen to you?
How it works
A normal sneeze starts when something irritates the lining of the nose. The trigeminal nerve carries that signal to the brainstem, which fires the sneeze3. In a photic sneezer, a sudden bright light does the same thing. Stepping out of a dark cinema into sun is the classic trigger. Looking at other bright lights works for some people too1.
The cause is not fully proven. The leading idea is cross-talk between nerves. The optic nerve, which carries light signals from the eye, runs close to the trigeminal nerve, which serves the face and nose24. A strong burst of light may leak signal into the wrong pathway. The brainstem reads it as an itch in the nose and sneezes. A related idea is that the visual part of the brain is more excitable in photic sneezers. A burst of light then also activates brain areas for touch5.
Most people sneeze two or three times per burst6. Sunglasses or a hat reduce the effect1.
The reflex is inherited. It is autosomal dominant, which means a child of one affected parent has about a 50% chance of having it1. A 2010 study using 23andMe data found gene variants linked to it1.
How common it is
Surveys put the reflex at 18% to 35% of people1. About one in four is the usual figure5. Many people who have it assume everyone does.
Is it a problem?
No. Sneezing in bright light is relatively harmless1. It is not caused by allergies. It becomes a practical concern mainly for pilots and drivers, since the sneeze briefly closes the eyes13. There is nothing to check with a doctor. Sunglasses or a hat before stepping into bright light can reduce how often it happens1.
History and culture
Aristotle asked why the sun makes people sneeze in the fourth century BC1. The acronym ACHOO, for autosomal dominant compelling helio-ophthalmic outburst, was coined as a joke and stuck1.
Sources and further material
- Wikipedia Photic sneeze reflex Eriksson and colleagues, 2010, found genetic variants linked to the reflex in a 23andMe study.
- Wikipedia Sneeze
- scientificamerican.com Looking at the Sun Can Trigger a Sneeze Scientific American. Says the trigeminal nerve is in close proximity to the optic nerve.
- NCBI When the sun prickles your nose: an EEG study identifying neural bases of photic sneezing PLoS ONE. Found a more excitable visual cortex in photic sneezers. Says about one person in four is affected.
- scientificamerican.com Why does bright light cause some people to sneeze? Scientific American. Says the sneezes typically number two or three.
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Related oddities
Pupillary light reflex
Shine a light into one eye and both pupils shrink, even the eye in the dark. The wiring crosses in the midbrain so that the two pupils react together. You can watch it in a mirror.
Gag reflex
Touch the back of the tongue or the soft palate and the throat clamps shut. It keeps things out of the airway. About a third of healthy people have no gag reflex at all, and that is normal.
Brain freeze
A short, sharp headache in the forehead a few seconds after cold food or drink touches the roof of the mouth. It usually passes in under a minute.
Knee-jerk reflex
A tap on the tendon below the kneecap makes the lower leg kick. The signal goes to the spinal cord and straight back through one connection. It is a classic example of a one-synapse reflex.