Picture this: You walk out the door on a bright, sunny morning. And before you’ve even reached the car, you sneeze. Maybe even two or three times in a row.

It’s not allergies, and it’s not a coincidence. It’s a real reflex, called the photic sneeze reflex.

By some estimates, it affects roughly one in four people.

Researchers have studied it for decades and have even given it a punny name: ACHOO syndrome, short for Autosomal Dominant Compelling Helio-Ophthalmic Outburst

What’s actually happening?

The leading theory is that the optic nerve, which senses that sudden shift in light, runs right next to the trigeminal nerve, which is responsible for sensory information from the face and head, including triggering a sneeze. 

When you move from a dim room into bright sunlight, your pupils constrict rapidly, and that burst of signal in the optic nerve appears to “spill over” into the neighboring trigeminal nerve. Your brain interprets that as an irritant in your nose, so you sneeze. 

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It’s worth noting that researchers aren’t entirely settled on this explanation. A competing theory suggests a broader parasympathetic response is at play, where light sets off a wider cascade of reflexes that happens to include sneezing. 

Either way, the trigger is well established: It's the sudden change in light.

Is it inherited?

The reflex tends to run in families, and earlier research suggested it might follow a straightforward inherited pattern, where if one parent has it, their child has roughly a 50% chance of inheriting it.

But a larger 2019 genetic study found that the picture is more complicated. The trait appears to involve many genes working together, rather than one dominant gene passed down in a simple fashion. 

(MORE: Your allergies may be worse in the fall)

This trait is not anything to generally worry about, mostly just a harmless quirk or sometimes an annoyance. But researchers have flagged one concern: a sneeze forces your eyes shut for a second, which could be a hazard for drivers emerging from tunnels into daylight, or pilots flying into open, sunny skies.  

So the next time you sneeze while walking out into the sun, you can rule out allergies, dust, or bad luck. Blame your nerves.

Miriam Guthrie is an associate meteorologist with weather.com who graduated from Georgia Tech with her undergrad and graduate degrees in Earth & Atmospheric Sciences.