Ever been caught in a sneezing fit when the dentist shines that bright light into your eyes? You’re not alone—lots of people experience the same strange phenomenon when they look at something bright.
There’s actually a name for this: the photic sneeze reflex. Or, if you want to have fun with it, call it the Autosomal Compelling Helio-Ophthalmic Outburst (ACHOO) Syndrome. This reflex affects somewhere between a fifth and a third of the world’s population—but what causes it? Let’s find out.
What Is the Photic Sneeze Reflex?
If this reflex continued the entire time you were in bright light, you might suffer. Millions of people would find themselves sneezing every time they spent a day in the sun. Luckily, the photic sneeze reflex only hits when you first encounter a bright light. Then, it goes away.
Normally, sneezing helps us clear out irritants from the nose. That’s why you sneeze more when you’re sick or have allergies. But the photic sneeze response clearly doesn’t serve this purpose—and scientists actually aren’t quite sure why it exists.
What we do know, however, is that it’s a genetic trait. The gene for this reflex appears near a gene that plays a role in light-triggered epileptic seizures, so there might be some connection between the two. However, because the photic sneeze reflex isn’t particularly common or harmful, it hasn’t been widely studied yet.
What Triggers This Response?
Light of any kind seems to trigger the photic sneeze response—or, more specifically, the transition from a dark area into a brightly-lit one.
Anything from stepping into the sun to turning on a lamp to seeing a camera flash can cause a photic sneeze. In fact, some people use the reflex on purpose to trigger a sneeze that they feel building up. It doesn’t matter what the light source is, as long as it follows a period of relative darkness: both natural and artificial light can cause it.
Many people can live with this reflex without ever realizing that they have it, because the occasional random sneeze is pretty normal. You can easily test this response on yourself and your friends by using a flashlight or any other handy light source.
Something about the way light stimulates the eyes causes people with this genetic trait to sneeze. Researchers haven’t yet figured out exactly what that something is.
Some believe the link could be found in the trigeminal nerve, which connects the eyes and the nose. It’s possible that light may stimulate this nerve in a way that creates the sensation of irritation in the nose. The nose interprets the nerve signal incorrectly, and makes you sneeze.
Or, it could be that the light causes tears in the eyes, which drain into the nose and may cause sneezing. One study, also linked the photic sneeze reflex to a deviated septum, suggesting that the causes may go beyond simply being genetic.
Although the photic sneeze response is mostly harmless, every once in a while it could spell danger. For example, a driver who starts sneezing when leaving a dark tunnel is more at risk of a crash. But luckily, if you want to combat this response, all you have to do is put on a pair of sunglasses.