“Sunshine on my shoulders makes me happy,” the late John Denver sang. “Sunlight in my eyes can make me cry.”
Lovely lyrics. But as a kid, I thought it would have made more sense for Denver to sing, “Sunlight in my eyes can make me sneeze.” Because for somewhere between one in 10 and one in three people, sunlight has exactly that effect.
It’s called “photic sneezing,” and it’s nothing new: Aristotle wondered about it in the 4th century BC (although he thought it was brought on by heat, not light). But millennia later, we still don’t know exactly why it happens, as New Scientist writer Richard Webb recently discovered.
The usual explanation for regular sneezing is that it serves to expel unwanted material from the airway. A regular sneeze begins with an irritation in your nose. This excites the trigeminal nerve, which sends impulses to the “sneezing center” in the brainstem, the primitive part of the brain that triggers our involuntary reflexes.
The sneezing center sends impulses along the facial nerve ordering the nasal passages to secrete fluid, and simultaneous impulses along the spinal cord to the respiratory muscles, prompting them to take a quick, deep breath (“Ah-”), then expel it with great force (a 150-kph “Choo!”). The abdominal, chest, vocal cord and throat muscles are all in on the act, as is your diaphragm and even your eyelids (you always close your eyes when you sneeze).
Sneezing is, as Webb notes in his article, “one of the most violent actions your body will ever perform,” so violent that cases of sneezing-caused whiplash are not unknown.
With today’s sensitive brain-scanning equipment, you’d think it would be a simple matter to track down the precise location of the “sneezing center.” But you’d be wrong. And since we can’t pin down the specific neurons involved, we can’t pin down how photic sneezing arises, either.
We do know a few things about it. In 1964 Henry Everett, a consulting psychiatrist at the Johns Hopkins University Hospital in Baltimore, Maryland, made one of the first systematic studies of the condition, questioning 75 of his patients and 169 of his students in detail about their sneezing habits. Among other things, he asked those with photic sneezing (18 percent of the patients and 24 percent of the students) if they had any close relatives who reacted to sunlight the same way, and found that 80 percent of the sneezers said they did, compared to only 20 percent of the non-sneezers.
This strongly indicated that photic sneezing has a genetic component, and further studies have borne that out. In fact, its inheritance is consistent with transmission via a dominant gene, meaning you only need one copy of it from either parent. This is known as autosomal dominant transmission, so photic sneezing now has the irresistible scientific name of “autosomal-dominant compelling helio-opthalmic outburst,” the acronym for which is, of course, ACHOO.
But that still doesn’t answer the question of why sunlight should make us sneeze, and as Webb found, at the moment nobody actually has an answer.
Not that there aren’t theories. Other things unrelated to nasal irritation can cause sneezing, after all. A study last year revealed there are patients who sneeze when they have an orgasm—or even simply in response to sexual thoughts.
Maybe all of these strange causes of sneezing come about because of a kind of short-circuit in the brainstem, where all kinds of reflex actions are triggered, so that various stimuli that trigger unrelated reflexes such as blood flow to the genitals or squinting against a bright light also trigger, quite by accident, a sneeze. The genes that create these short-circuits are more nuisances than threats to survival, and so have been preserved by evolution.
Which sounds plausible, but there isn’t actually any solid evidence for it. Or as Webb quotes Louis Ptácek, a neurogeneticist at the University of California in San Francisco, as saying, “People speak as if they know what the hell’s going on. In reality, we don’t.”
As our tools for studying the brain improve, maybe we’ll figure it out. In the meantime, if sunlight makes you sneeze, try to follow Aristotle’s example:
Just be philosophical about it.