You step out of a dim house or a dark movie theater into bright sunshine, and suddenly your nose tickles. Then comes a sneeze, and maybe another one right behind it. If that sounds familiar, you're far from alone.
It's called the photic sneeze reflex, an inherited trait that makes some people sneeze when they're suddenly exposed to bright light. It's thought to affect somewhere between 18 and 35 percent of people worldwide. Scientists have a few theories about why it happens, but the exact cause is still not well understood.
A Name Made for a Laugh
The reflex has a playful nickname, ACHOO syndrome. That's a contrived acronym for Autosomal-dominant Compelling Helio-Ophthalmic Outburst. Somebody clearly worked hard on that one.
The silly name hides a real clue. Autosomal dominant means the trait is passed down in families, so a parent who has it has a good chance of passing it on to a child. Researchers have even found a specific genetic variant that seems to raise the odds, though they don't yet know how it works.
Aristotle Wondered About It, Too
People have been puzzling over sun sneezes for a very long time. Around 350 BCE, the Greek philosopher Aristotle asked, "Why does the heat of the sun provoke sneezing?" He guessed that the sun's heat made the inside of the nose sweat, and the sneeze cleared out the moisture.
In the 1600s, the English philosopher Francis Bacon tested that idea by facing the sun with his eyes closed. No sneeze. He concluded that the eyes must be involved, and guessed that bright light made the eyes water, with the moisture trickling into the nose. Scientists later ruled that out too, because the sneeze happens too fast for watery eyes to be the trigger.
Crossed Wires in the Face
A normal sneeze starts when something irritates the inside of your nose. That signal travels along the trigeminal nerve, a major nerve of the face, to the brainstem, which then fires off the sneeze.
One leading idea is that the nerves serving the eyes and the nose sit close enough together that a strong signal from bright light spills over and sets off the sneeze pathway by mistake. Another theory says that when one set of nerve fibers gets activated, neighboring fibers can get swept along. Either way, it's a bit like a crossed wire.
There are hints that this face nerve is simply extra touchy in people with the reflex. In many of them, poking the nerve directly, for example by plucking an eyebrow or pulling a hair, can set off a sneeze too. Researchers are still debating which explanation, or which mix of them, is right.
When It Matters
The reflex seems to be triggered by a change in brightness rather than a particular color of light, and it tends to be stronger after you've spent time in the dark. For most people, it's a harmless annoyance.
It can matter in certain situations, though. A sudden sneezing fit while driving into the sun could make someone lose control, and the reflex is considered a real concern for pilots facing bright sunlight and reflections off water. If you sneeze a lot in bright light and it worries you, talk to your doctor.
Otherwise, consider it a little family heirloom. The next time you sneeze at the sunshine, you can say you share a mystery that stumped Aristotle himself.
