• It's a basic reflex to make you close your eyes.
  • An excellent question. This is far more complicated than you might imagine. Since the detailed answer below got kind of long, I will summarize the main points up front. About 25% of people do actually sneeze when exposed to bright lights like the sun. We do not know exactly why this happens, but it might reflect a "crossing" of pathways in the brain, between the normal reflex of the eye in response to light and the sneezing reflex. There is no apparent benefit from "sun-sneezing", and it probably is nothing more than an unimportant (but annoying) holdover of evolution. There are actually three questions here: 1) Do people really sneeze when they look at the sun? It seems that some people really do sneeze when they look at the sun, or actually at any bright light (there is nothing special about the sun). This has been recognized in medical journals for at least 40 years, and is called different things, including the "photic sneeze reflex" and even the "ACHOO syndrome". In a few instances the reflex has been documented by shining bright lights at people to make them sneeze, so it probably really does happen. The thing is, the sun does not make most people sneeze. The best estimates are that only about 25% of people (i.e. 1 in 4) are sun-sneezers, and even sun-sneezers don’t do it most of the time. Other studies suggest that sun-sneezers might have inherited the trait in a dominant fashion from their parents (which means that if only one parent was a sneezer, they would pass the trait on to half of their kids). The problem with these estimates is that they generally rely on asking people if they sun-sneeze, and if their parents do. This is VERY unreliable. People who sun-sneeze tend to assume everyone does, and people who don’t sun-sneeze have often never even considered the possibility!! 2) What is the mechanism by which sun-sneezing occurs? What actually makes it happen? The simplest answer is that we don’t really know. There are many theories that I won’t get into specifically, since most require a detailed understanding of brain anatomy to even describe. What I will try to do instead is to briefly explain the nature of the sneeze reflex and offer a simple understanding of how light might act as a trigger. What is a sneeze? Well, everybody knows what a sneeze is, but try and describe it sometime! The fact is, a sneeze is a very complicated thing, involving many areas of the brain. A sneeze is a reflex triggered by sensory stimulation of the membranes in the nose, resulting in a coordinated and forceful expulsion of air through the mouth and nose. A "reflex" means that some type of stimulation of your body causes you to react in a way that is NOT under your control, in other words you do it automatically without thinking and you can’t even stop it. Your body has many reflexes - the other one important to us here is called the "pupillary light reflex". If you shine a light in your eyes, your pupils get smaller, or constrict. In the pupillary light reflex, shining a light in the eye causes nerve signals to go from the eye to the brain and then back the eye again, telling the pupil to constrict. In the usual sneeze reflex, tickling the nose causes nerve signals to go from the nose to the brain and then back out to the nose, mouth, chest muscles and everything else involved in the actual sneeze. The key point is that the nerve signals take complicated routes through the brain, but usually the pupillary light reflex and sneeze reflex take different routes. Apparently what happens in sun-sneezers is that shining a bright enough light in the eye ALSO sends nerves signals from the eye to the brain and then back out to the nose, mouth and chest! In short, the wires are crossed a little bit in some people, and so shining a light in the eye "accidentally" activates two different outgoing pathways. 3) What is the benefit or value of sun-sneezing? Is it a good thing? Again, not an easy question! Some people argue that it must be a good thing since the trait has been maintained in our population. I’m not so sure. What is the benefit of sneezing in general? The easy answer is that the thing tickling your nose might be a bad thing (like a virus, bacteria or pollen), and sneezing forces it out. The problem is that most adults sneeze mostly through their mouth, and so sneezing won’t force anything out of the nose. This is in contrast to most animals, who sneeze largely through their nose (watch your dog or cat next time). So I think that sneezing really serves little purpose to humans (little kids might be an exception to this). I think that sneezing is important to animals that rely heavily on the sense of smell, but that in us it is just an annoying "holdover" of evolution. But that is just my opinion. But even if a normal sneeze does function in ridding the nose of foreign materials, I can think of no reason why shining a light in the eye would create a need to force things from the nose! And in the several papers I have read on this subject, no one else gave a reason why sun-sneezing would be beneficial. Sun-sneezing must truly either be an accident of brain anatomy, or an evolutionary holdover from our animal friends, that serves no purpose in humans. Thanks for a very thought-provoking question. Keep ‘em comin’!

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