The Reflex That Will Not Take Instructions
A sneeze is one of a very small number of things the body does that a person can feel approaching, can do absolutely nothing about, and cannot start on purpose. That combination is rarer than it sounds. Most reflexes are either entirely unconscious, so there is no warning, or partly voluntary, so they can be overridden.
The sneeze sits awkwardly in between. There is a clear sensory build-up, usually several seconds of it, during which the person knows precisely what is about to happen. And yet past a certain point in that build-up there is no stopping it, because the decision has already been handed over to a cluster of cells in the brainstem that does not accept further input.
The reverse is equally true and equally odd. Almost nobody can produce a real sneeze deliberately. People can imitate the sound and the motion, but the actual reflex, with its particular sequence of breath, closure and release, cannot be summoned to order. A great many people who want to sneeze and feel one stalling resort to looking at a light, which is a clue to the subject of this piece.
So the sneeze is a thing the body does to a person rather than something the person does. That is worth holding on to, because it shapes everything that follows, including why the strangest trigger of all is so hard to explain.
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What Is Actually Happening, in Order
The usual starting point is an irritant somewhere in the lining of the nose. It does not have to be a particle. It can be a chemical, a sudden change in temperature or humidity, or inflammation from an infection or an allergy. What matters is that nerve endings belonging to the trigeminal nerve, which supplies sensation to the face, are stimulated.
That signal travels to the brainstem, where it is gathered together with everything else arriving at the same time. If enough of it accumulates, a threshold is crossed and a fixed motor programme runs. The programme is fixed in the sense that it always produces the same sequence, which is why sneezes resemble each other far more than, say, coughs do.
The sequence has two parts. First comes a deep inhalation with the vocal folds held open, drawing a large volume of air into the chest. Then the folds close, the chest and abdominal muscles contract hard against that closure, pressure builds, and the folds open suddenly. The air that has been compressed behind them leaves at high speed, and it leaves through the nose and mouth together because the soft palate is positioned to allow both.
Several other things happen in the same programme without being separately decided. The eyes close. The head moves forward. In many people the whole upper body jerks. None of these is optional, and the eye closure in particular is simply part of the package rather than a protective measure anybody arranged.
Why the Spread Is So Much Greater Than It Looks
The visible part of a sneeze is a small spray that seems to fall quickly. What actually leaves is considerably more complicated.
A sneeze produces droplets across an enormous range of sizes, from ones large enough to see and heavy enough to fall within a metre or two, down to ones so small that they behave less like falling objects and more like smoke. The small ones do not settle in any meaningful sense. They stay suspended, drifting with whatever air movement is in the room.
They also do not travel alone. The whole exhalation leaves as a warm, moist, turbulent cloud of gas that carries its droplets along inside it, and that cloud holds together for far longer and travels far further than any individual droplet would on its own. The droplets are not launched; they are transported. That distinction is why measurements of how far a sneeze reaches came out so much larger once people studied the gas cloud rather than the droplets.
This has nothing to do with the mystery in the title, but it is worth including because it explains why a sneeze is such an effective biological event. It is a very efficient mechanism for getting the contents of one nose into the air of an entire room, which is useful if you are a virus and inconvenient if you are everybody else.
The Trigger That Has Nothing to Do With the Nose
Now the strange part. A substantial minority of people sneeze when they move suddenly from dim light into bright light. Walking out of a building into sunshine will do it. So, for many of them, will a sudden bright indoor light.
Estimates of how common this is vary with how the question is asked, but the figure generally lands somewhere between a sixth and a third of people, which makes it one of the most common human traits that most people have never heard a name for. It is strongly familial. If one parent has it, a child has a substantially raised chance of having it, and the pattern across families is consistent with a single dominant inherited factor, though the picture is almost certainly more complicated than that.
Some features of it are very consistent. It is the change in light that matters, not the absolute brightness, so it happens on stepping out and not after ten minutes of standing in the sun. The number of sneezes tends to be fixed for a given person, so somebody who sneezes twice will usually sneeze twice every time. And it is instantaneous rather than building up over several seconds the way an irritant sneeze does.
The trait has been noticed for a very long time. Ancient writers recorded that looking at the sun made people sneeze and disagreed about whether the heat or the light was responsible, which, as it turns out, was a reasonable thing to disagree about.
The Leading Explanation, and the Hole in It
The usual account rests on anatomy. The nerve that carries sensation from the nose and the nerve that carries visual information from the eye are different nerves, but the pathways that handle them run close together on their way into the brain, and the pupil-constricting response to bright light involves a region near where facial sensory signals are processed.
The proposal, then, is cross-talk. A sudden strong signal in the visual pathway spills over, is picked up by neighbouring circuitry, and is read by the brainstem as if it were an irritation signal from the nose. The sneeze programme then runs as normal, because the programme has no way of knowing where the trigger really came from.
It is a tidy story and it has one obvious problem: it predicts that almost everybody should have the trait, because almost everybody has the same anatomy in roughly the same arrangement. The explanation accounts for the mechanism but not for the distribution, and the distribution is the thing that actually needs explaining.
Attempts to fill that gap generally propose that the difference lies in how excitable the relevant circuits are, so that some people’s threshold is low enough to cross and others’ is not. That is plausible and is consistent with the trait being inherited. It is also, at present, closer to a restatement of the observation than to an independent explanation, which is an honest place to leave it.
Studies looking at brain activity during the reflex have found differences in the responses of people who have it, which supports the general shape of the cross-talk idea. What has not been produced is a demonstration of exactly which connection is responsible and exactly what varies between people.
Other Triggers That Also Come From Nowhere
The light response is the famous one, but it is not alone. There is a well-documented tendency in some people to sneeze after eating a large meal, which appears to involve the same nerve supply reacting to stretching of the stomach. There is a reported association with plucking hairs from the eyebrow, which would fit the pattern of a facial sensory nerve branch being stimulated somewhere unexpected and the signal being misread further upstream.
Cold air does it to a great many people. So does a sudden strong smell that is not irritating in any chemical sense. And a sneeze that has been building and then stalls can frequently be completed by any sharp sensory input at all, which is the practical observation behind the folk habit of looking at a light.
What all of these have in common is that the sneeze programme is being started by something other than a particle in the nose, and the brainstem is running the programme anyway because it responds to a quantity of incoming signal rather than to an identified cause. That is a design choice with an obvious benefit, since it means a really irritating substance of a kind the body has never encountered will still provoke a sneeze. The cost is a reflex that fires at sunlight and dinner.
Why You Do Not Sneeze in Your Sleep
People essentially do not sneeze while properly asleep, and this surprises almost everybody who hears it. It is not that the nose stops being irritable. It is that the motor neurons responsible for the sneeze are suppressed during sleep, along with most of the rest of the body’s voluntary muscular apparatus.
The sensory side still works. A strong enough irritant will wake a person, and then they will sneeze, which is why anybody with a cold is convinced they sneeze all night. What is actually happening is a wake-then-sneeze sequence rather than a sneeze during sleep.
This is one of the clearest demonstrations that the sneeze is a whole-body motor event rather than a local nasal one. A local reflex, like the eye watering, carries on perfectly well while a person is asleep. The sneeze does not, because it requires the coordinated recruitment of the diaphragm, the chest wall, the abdominal muscles and the larynx, and that machinery is switched down.
What It Is Unambiguously Good At
Whatever is going on with sunlight, the core function is not in doubt. The nose is the body’s main air intake and it is lined with a moving carpet of mucus that traps particles and carries them steadily backwards to be swallowed. That handles the routine load.
The sneeze is the emergency option for when that system is overwhelmed or when something arrives that must not be allowed further in. It generates, in a fraction of a second, a flow fast enough to strip material off the lining and expel it, and it does so without requiring any decision from anybody. For an organism whose airway is also its food pipe and whose nose is permanently open to the world, that is a reasonable thing to have.
The oddity is that a mechanism this specific and this well-defended should also be reachable by a bright sky. It suggests that the brainstem’s threshold detector is less fussy about the origin of its inputs than the elegance of the output would imply, and that a reflex can be very good at its job and still be wired to something it has no business responding to.
A Fixed Programme With a Loose Trigger
So the sneeze is a sealed sequence that cannot be started deliberately and cannot be stopped once it has begun, that expels a turbulent cloud travelling much further than it appears to, that is switched off entirely during sleep, and that in a sizeable fraction of people can be set off by nothing more than walking outdoors on a clear morning.
The mechanism of the reflex is thoroughly understood. The trigger that most people have personally experienced is not. That is an unusual combination, and it has survived two millennia of people noticing it and writing it down.
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