Why Does the Sun Make Me Sneeze? Here’s What to Know About ACHOO Syndrome

Key Takeaways on ACHOO Syndrome

• Some people sneeze when they look into the sun because of Autosomal Compelling Helio-Ophthalmic Outburst syndrome (ACHOO), or photic sneeze reflex (PSR). It happens when the brain misinterprets a powerful signal (sunlight) from the optic nerve and activates the sneeze pathway.
• About 10 percent of the general population can experience this syndrome.
• For most people, the sun sneeze is a harmless reaction, solvable with sunglasses. However, a photic sneeze could distract a combat pilot during high-stakes maneuvers. Similarly, exiting a dark tunnel while driving could lead to an uncontrollable reaction, so caution is key.

Ever left a dark theater or a dimly lit room, and suddenly sneezed? There’s no dust or pepper, just the sunlight that greets you when you exit. You may be surprised to learn that wanting to understand “sun sneezing” is more than just a simple curiosity, and that researchers are using this visual quirk to shed light on the intricate wiring of our brains.

The neurological impulse’s proper name is the photic sneeze reflex, or PSR. It also has a rather perfect acronym: ACHOO syndrome, or Autosomal Compelling Helio-Ophthalmic Outburst. Despite its peculiar nature, PSR is quite common.

“We think that this is about 10% of the general population,” says Dr. Louis Ptáček, a neurologist at the UCSF Weill Institute for Neurosciences. This prevalence of the reflex may be even higher, with some estimates ranging as high as 30 percent of people worldwide.

Why Does the Sun Make Me Sneeze?

The leading theory behind PSR’s cause involves a case of “crossed wires” in the brain. In a typical sneeze, the trigeminal nerve – the main sensory input for the face – detects an irritant in the nose and fires an initial signal. However, in a photic sneeze, researchers believe the brain misinterprets a powerful signal from the optic nerve and mistakenly activates the sneeze pathway.

The command then travels down to the diaphragm, which controls our breathing. “The nerve that innervates the diaphragm is the phrenic nerve,” Ptáček says. “Light signals coming in cross wiring, sending an output to the phrenic nerve and inducing a sneeze.”

Brain recordings suggest that there’s more to this response than meets the eye. A 2010 EEG study published in PLoS One found that photic sneezers showed stronger responses in their brain’s visual cortex to bright flashes, demonstrating that the reflex involves higher, more complex neural circuits than a simple brainstem reaction.

Read More: Why Do We Sneeze?

How Common Is ACHOO Syndrome?

Dr. Ptáček notes that the reflex appears to be an inherited trait. “One of the parents frequently has it,” he says. “Half of the children are likely to have it, half the siblings. So, it looks like it’s being transmitted as a dominant trait.”

In genetic terms, this means you only need to inherit PSR from one parent to express it. While the inheritance pattern is strong, pinning down a single culprit gene has been tricky.

“You can have these different mutations coming in from different sides of the family,” says Ptáček. This suggests that several genetic variants influence PSR, rather than a single on-off switch.

To hunt for clues, scientists used a method called genome-wide association study, or GWAS. They scanned the DNA of numerous individuals to find small differences in gene sequences associated with a target trait. A 2019 paper published in Scientific Reports did just that, surveying over 3,400 people and flagging several variants, or markers, tied to PSR across populations.

Interestingly, the literature notes that these markers are “intergenic,” meaning they are in the stretches of DNA between genes, not within them. These sequences may act more like dimmer switches, regulating the activity of nearby genes rather than directly changing their function.

How ACHOO Impacts the Body

Dr. Ptáček’s interest in the photic sneeze reflex comes from his work on more serious genetic disorders like epilepsy and migraine. Like PSR, these conditions are episodic, meaning symptoms occur periodically, often due to sensory inputs. Therefore, understanding the basis of PSR, Ptáček says, “might shed light on better treatments for other episodic diseases where we see reflex phenomena, like in epilepsy.”

The logic is bolstered by a tantalizing clue. A 2020 study published in the Journal of International Medical Research identified the ZEB2 gene, located right next to one of PSR’s genetic markers, as a potential prime target for epilepsy treatment.

Thoroughly cataloging PSR’s “phenotype,” or its specific characteristics, would be a key next step toward unraveling its mystery. These experiments would involve studying how long a person needs to be in the dark, and how bright the ensuing light needs to be, to trigger a sneeze.

For Ptáček, the value of this research is clear. “I believe that discovery and understanding biology is worthwhile all the time,” he says. “If we’re learning something new about the human body, that’s cool in itself, but it will also be valuable in treating people with diseases.”

Is Sneezing from the Sun Dangerous?

For most people, the sun sneeze is a harmless reaction, solvable with sunglasses.

“People with photic sneeze don’t have any other problems that we recognize,” Ptáček says.

There are, however, potential rare cases. A 1990s study in Military Medicine warned that a photic sneeze could distract a combat pilot during high-stakes maneuvers. Similarly, exiting a dark tunnel while driving could lead to an uncontrollable reaction, so caution is key.

While the condition may be benign, the millions born with PSR are living examples of one of the brain’s more interesting shortcuts: the tiny reflex that may just help us answer some of neurology’s biggest questions.

This article is not offering medical advice and should be used for informational purposes only.

Read More: Everything to Know About Allergies

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article: