Do you or one of your friends sneeze when you look at the light? Then, you’re in good company. Here’s what you need to know.

Not to brag but Aristotle and I have something in common. I didn’t make important and pioneering contributions to the fields of science and philosophy, nor did I mentor Alexander the Great, one of the greatest-known men of all time. Rather, Aristotle and I share a similar (and peculiar) affliction; we’re both burdened sufferers of the dreaded sun sneezes — also known as photic sneezes.

Sun sneezes or photic sneezes aren’t just an innocent case of the sniffles. They aren’t seasonal allergies or caused by viruses like the rhinovirus; they’re literally caused by just looking at bright light. You can’t get rid of them through medication either. This makes them pretty annoying (on most occasions) and hard to understand for people who don’t have this problem.

In fact, way back in Ancient Greece, sun sneezes annoyed Aristotle so much that he actually put pen to paper to rant about it. Imagine taking a break from formalizing fields of study to write about sneezing. Well, that’s exactly what Aristotle did.

“Why does the heat of the sun provoke sneezing, and not the heat of the fire?,” he inquired in The Book of Problems. Although we haven’t yet found an answer to all of his questions, a fair bit of work has been done on this particular inquiry. For starters, we now know that sneezing when you look at the sun isn’t that common.

What are sun sneezes?

Aristotle was right about many things, but he was a bit off on this one. It isn’t the heat of the sun that causes sun sneezes; it’s the light.

It’s also rather uncommon to get photic sneezes. If you (like me) assumed that everyone forces out sneezes by staring at the sun till they either blind themselves or successfully sneeze, I regret to inform you that you and I are outliers.

Turns out, only about a third of the world’s actually sneezes when they see the sun. Everyone else? they find out when they look at us: weirdos who run around looking for sunlight when they need to sneeze. In fact, it doesn’t just have to be sunlight. Literally, any source of light, if bright enough, elicits the same response (the achoos!). This odd tendency is scientifically known as a photic sneeze response.

It took quite a while for sun sneezes to get a “formal” term. It wasn’t until the 1950s, that someone finally investigated sun sneezes, and what caused them. Sedan, a French researcher, noticed a few of his patients oddly sneeze in response to exposure to the bright light of the ophthalmoscope — the hand-held scope eye doctors use to examine patients’ eyes. Although Sedan came up with significant findings (for instance, he figured out that the sneezes happen just as patients were exposed to light — not for the entire period of time they were exposed to light) it wasn’t until 1964 that the term “photic-sneeze reflex” came to be.

The latter half of the twentieth century seemed to be a booming time for sun sneezes. Not only was the odd reaction finally formalized in literature in the 60s but just again in the 70s, it got another name – ACHOO!

What is ACHOO?

The confusingly formulated acronym stands for Autosomal Dominant Compelling Helio-Ophthalmic Outburst Syndrome. Yes, it’s a backronym, an acronym deliberately formed from a phrase whose initial letters spell out a particular word or words.

To understand what the whimsical term stands for, we just need to understand the first letter of it. Basically, the A in ACHOO stands for autosomal dominant — a genetic term used to denote a pattern of inheritance. Essentially, if you sniffle when you see the sun, your kids have a 50 percent chance of doing the same. Since the ACHOO trait is heritable, you can even pick it up on DNA tests like 23AndMe.

What causes the photic sneeze reflex?

Dust and germs are the things that usually cause sneezes, not light. So why does a third of the population sneeze when they’re exposed to bright light? Well, there are a lot of theories out there but the most popular one is that it’s a case of faulty circuitry in the brain.

The head’s a crowded place — and that’s without intrusive thoughts. Apart from the brain, the ears, the mouth, and the nose, the head also houses 12 pairs of nerves known as cranial nerves. Think of it like this, the senses perceive external information (like taste and smell) and the nerves transmit this information to the brain. Not only are these nerves important, they’re also placed incredibly close to each other.

The most popular theory when it comes to photic sneezing involves these nerves, more specifically the optic nerve — the second cranial nerve — and the trigeminal nerve — the fifth cranial nerve. The optic nerve handles sight and the trigeminal nerve handles smell but sometimes both get their signals crossed (literally). So sometimes, instead of looking at light and processing it as just visual information, our brain may also process it as a need to sneeze as well.

Although prevalent, the theory is yet to be confirmed. According to Louis Ptáček, a geneticist at the University of California, San Fransico, not a single theory can yet fully explain sun sneezes without more in-depth studies.

When Speaking to PBS Newshour, Ptáček opined that there’s a lack of willingness to fund research in relation to sun sneezes.

“It’s hard to get funding because reviewers don’t think of it as a problem,” he said. “Instead, money goes to research on diseases like Parkinson’s and multiple sclerosis, ” explained Ptáček in the same article.

Other possible theories about the sun sneeze

Quite a few theories have attempted to explain the photic-sneeze reflex. For instance, a particular theory tried to explain the autosomal trait as a consequence of evolution. The theory posited that back when there were cavemen who lived in unaesthetic and dusty caves, sneezing as a reaction to light helped them detox their noses if needed.

Another theory came about as the result of a 2010 study conducted by researchers from the University of Zurich, Switzerland. This study revealed that photic sneezing isn’t just a classical reflex arc — like the classical knee-jerk. Rather, the study found that individuals prone to photic sneezes had highly sensitive visual cortices — a part of the brain at the back of the head. This means that whatever triggers the sneeze reflex isn’t just basic electrical activity at the spinal cord level but rather a concentrated process that engages particular parts of the brain.

Most studies and theories conducted so far have one thing in common: they’ve all pointed out a need for more in-depth research. In the PBS Newshour article, Ptáček elaborated on the possible significance of a breakthrough in the identification of the neural pathways involved in photic sneezing.

“If we knew one or more genes that cause photic sneeze reflex, I don’t doubt that that might teach us fundamental things about reflex disorders like epilepsy…some of the most important advancements in medicine come from not being focused on medicine at all,” explained Ptáček.

How common are sun sneezes?

Sun sneezes are odd in more ways than one. They’re not rare but that doesn’t make them common either. In 2016, a team of Spanish researchers was able to determine that only about 10 to 35 percent of the world’s population is prone to photic sneezes. As a matter of fact, not only is the reflex uncommon; but it’s also largely unheard of. Most photic sneezers might not even know that they’re photic sneezers.

Photic sneezing isn’t harmful. It won’t land you in the ER by any means however depending on your occupation, photic sneezing might be a big deal. For instance, if you’re a businessman or consultant, photic sneezes probably won’t affect your life too much. On the flip side, if you’re a long-distance trucker or a combat pilot, photic sneezes might just be the difference between life and death.

Imagine, flying a fighter jet that whizzes through the air at speeds of thousands of miles an hour, as intense sunlight literally beams down right at you. Similarly, imagine driving an 18-wheeler out of a dark exit into a bright light. The last thing you’d want to do in either of these situations — ACHOO trait or not — is sneeze.

The bottom line on photic sneezes

In conclusion, the world of sun sneezes—known scientifically as the photic sneeze reflex or colloquially as ACHOO—is more complex and fascinating than one might initially think. From Aristotle’s ruminations in Ancient Greece to modern genetic studies, this quirky bodily response has perplexed and fascinated us for centuries. While not inherently harmful, photic sneezing has its caveats, especially if your profession demands keen attention and reflexes, such as piloting or long-haul driving.

If you’re not a tightwalker, F1 driver, or a Top Gun fanatic who contemplates joining the Naval Air Force every time they listen to Take My Breath Away, photic sneezes will not kill you. On the contrary, if you’re like me, you might just adopt a glass-half-full approach to photic sneezing. Because, you, like me (and let’s not forget: Aristotle) have a pretty cool party trick: there’s no sneeze in the world we can’t force out.

The photic sneeze reflex offers an interesting point of convergence for different fields—neuroscience, genetics, evolutionary biology—each providing a piece to a puzzle that is far from complete. And who knows? Unlocking the secrets behind a seemingly trivial sneeze could lead to groundbreaking research that helps us understand more serious neurological conditions.