A sneeze is one of the few bodily functions capable of throwing a dignified adult into temporary chaos. One moment, you’re mid-sentence; the next, your face contorts, your lungs compress and your entire nervous system commits to launching microscopic droplets across the room at startling speed.

This ancient biological reflex is shared, in various forms, with many other animals. Long before humans understood viruses or allergies, organisms still needed a way to protect their delicate respiratory tissues from dust, pathogens, parasites, smoke, pollen and whatever else the environment happened to throw at them. Sneezing appears to be evolution’s solution to this — and while it may be abrupt and inelegant, it’s still remarkably effective.

Yet despite its ubiquity, scientists are still in the process of uncovering the finer details of how sneezes work and why they evolved in the first place. Here’s what modern research has revealed about why we sneeze, why our eyes snap shut when we do and why not every creature on Earth is capable of the reflex.

The Sneeze Is An Emergency Air-Clearing Mechanism

Sneezing is primarily a defensive reflex. According to a 2025 review in Frontiers in Neuroscience , sneezing begins when specialized sensory receptors inside the nasal lining detect irritation. These irritations, as many will know, come in a variety of shapes and sizes: dust, pollen, pepper compounds, cold air, viruses, inflammatory chemicals or even mechanical stimulation from a stray particle lodged in the nose.

Those signals travel primarily through branches of the trigeminal nerve (one of the major sensory nerves of the face) toward networks in the brainstem, which are sometimes referred to collectively as the “sneeze center.” Then, once activated, the brain rapidly coordinates a full-body response.

First comes a deep inhalation. Then the glottis closes, which briefly traps air inside the lungs while the chest and abdominal muscles contract; in turn, pressure builds. Finally, the glottis bursts open, sending air explosively through the nose and mouth at tremendous speed — almost 100 kilometers per hour (62 miles per hour), according to the 2025 review.

This sequence unfolds in only a few fractions of a second, and it involves astonishing coordination between respiratory muscles, facial muscles, the throat and autonomic nervous system pathways. And while sneezing may feel somewhat chaotic from the inside, neurologically speaking, it’s a highly organized and well-coordinated event.

For decades, scientists assumed the primary purpose of sneezing was simply to expel irritants from the upper airway. This explanation still seems to hold true: sneezing physically ejects mucus, particles, microbes and allergens before they can penetrate deeper into the respiratory tract.

However, newer hypotheses suggest, as noted in the 2025 review, that the reflex may also serve to redistribute mucus across the nasal cavity; it’s believed that this enhances the trapping and dilution of harmful particles, rather than just blasting them outward. This means that sneezing may function partly as a maintenance mechanism for the nose’s broader immune defenses — which makes it slightly less gross and a little bit more impressive.

Why Your Eyes Slam Shut When You Sneeze

Almost everyone has heard the myth that your eyes close during a sneeze to stop them from popping out. But anatomically speaking, this is absolute nonsense. Your eyes are very firmly anchored within the orbit by muscles, connective tissue and surrounding fat. A sneeze doesn’t generate nearly enough internal pressure to eject them like champagne corks.

So, if this explanation falls flat, then why do our eyes close? A 2007 review in Current Allergy and Asthma Reports suggests that the answer ties back to the sneeze reflex itself, specifically in how it recruits multiple cranial nerve pathways simultaneously. When the sneeze circuitry activates in the brainstem, the respiratory muscles aren’t the only ones your body coordinates; it also activates facial muscles. These include the orbicularis oculi, the muscles responsible for shutting the eyelids.

This means that eye closure may actually be part of the sneeze program itself. Researchers believe this may serve a protective function. Sneezing forcefully expels mucus and airborne debris; reflexive blinking may help shield the eyes from contaminants moving upward across the face. But that’s likely as far as the protective mechanism goes; the eyelid closure isn’t keeping your eyes from shooting out of the socket.

But overall, the simplest neurological explanation is that once the sneeze reflex begins, neighboring motor pathways become broadly activated. This is likely what produces the characteristic facial scrunching associated with sneezing.

What few people know, however, is that eye closure isn’t completely mandatory. Some people can keep their eyes partially open during a sneeze, particularly if they consciously try. This tells scientists that the reflex is strongly linked, but not absolutely hardwired beyond voluntary control. But still, most of us lose that battle instinctively.

Why Bright Sunlight Makes Some People Sneeze

For some people (about 25% of the population), stepping into bright sunlight can trigger a sneeze attack. This phenomenon is formally known as the photic sneeze reflex, or as an Autosomal Cholinergic Helio-Ophthalmologic Outburst — unofficially blessing it with the spectacular acronym ACHOO syndrome.

For a while, scientists assumed that this reflex was a simple case of crossed wires somewhere deep in the brainstem: light stimulates the eyes, nearby sneeze circuitry accidentally gets activated and out comes a sneeze. Then, in a 2010 study published in PLOS One , researchers uncovered the real neural bases of the phenomenon.

In the experiment, the study’s authors exposed both “photic sneezers” and non-sneezers to visual stimuli, including flashing lights, while recording their brain activity using EEG scans. Overall, they discovered measurable differences between participants in how the brain itself processed visual information.

Compared to control subjects, photic sneezers showed heightened excitability in the visual cortex, particularly in a region called the cuneus, which helps process incoming visual stimuli. This suggests that photic sneezers are unusually sensitive to bright light from the very earliest stages of visual processing.

Moreover, participants who reported stronger prickling sensations in the nose during light exposure also showed increased activation in the insula and secondary somatosensory cortex, two brain regions that are involved in bodily sensation and sensory integration. This means the brain may be blending visual input with physical sensory experiences in ways that do not normally occur.

For this reason, the researchers proposed that the photic sneezes aren’t as reflexive as previously thought. More likely, it involves higher-order cortical regions responsible for processing vision and bodily sensation together. If it were truly a classical reflex (like the knee-jerk reflex at a doctor’s office) then it would operate largely without conscious cortical involvement. But the photic sneeze seems to recruit broader sensory networks in the brain itself, so this cannot be the case.

In practical terms, this means that some people may sneeze in sunlight because their brains are unusually responsive to visual stimulation, and because that heightened visual activity co-activates sensory areas associated with irritation in the nose and face.

Humans are far from being the only sneezers on the planet. Many mammals sneeze, including dogs, cats, horses, primates and rodents. Birds exhibit sneeze-like respiratory expulsions as well, and even some reptiles do, too. Across these groups, the function appears broadly similar: clearing irritants, pathogens, mucus, parasites or excess secretions from respiratory passages.

But not all animals sneeze, because not all animals possess the anatomical machinery required for sneezing in the first place. This is because a true sneeze generally requires several components:

  • A respiratory system that moves air under pressure
  • Sensory structures capable of detecting irritation
  • Nervous system pathways coordinating the reflex,
  • An airway through which material can be forcefully expelled

Fish, for instance, don’t sneeze in the mammalian sense. Although they do have nostrils, these structures are usually involved in smell rather than breathing. Fish move water across gills rather than air through lungs and nasal passages. Some species can forcefully expel water or debris, but this isn’t considered a true sneeze.

Insects are even further removed. Their respiratory system consists of spiracles and branching tracheal tubes that deliver oxygen directly throughout the body. There is no centralized nasal airway to clear, and no evidence of a sneeze reflex comparable to that seen in vertebrates.

From an evolutionary perspective, this tells us that sneezing is closely tied to life on land and the challenges of breathing air filled with particles, spores, smoke, allergens and microbes. Once organisms evolved pressurized air-breathing systems, they also needed mechanisms to defend them. For all its drama, sneezing may just be one of evolution’s oldest forms of housekeeping.

Your sneeze has an evolutionary backstory millions of years in the making. How deep does your knowledge of evolution actually go? Take my fun Evolution IQ Test to find out.