The Surprising Reason Some People Sneeze When They Look At The Sun

A very annoying feeling is the need to sneeze but the inability to actually make it happen. Older people might recall being told to look up at the sun when they encounter this frustration. The idea that looking at the sun will help one sneeze is perhaps typically thought of as an old wives' tale, so it might be surprising to learn that they were relying on some internal wisdom. This advice is actually based on a real neurological phenomenon with a funny name: ACHOO (autosomal compelling helio-ophthalmic outburst) syndrome. It occurs when individuals sneeze in response to sudden brightness. This is also known as the photic sneeze reflex.

Does this response happen to everyone? No, but ACHOO syndrome might be more common than one would think. Much like the science behind physical traits passed down from parents, researchers have found that the photic sneeze reflex is an inherited, dominant trait. Meaning, if someone has a parent who exhibits this quirk, they have a 50% chance of having it as well. Results vary, but a study from 2010 suggests that one in four people have this response.

Paying attention to old wives' tales is not a fruitless endeavor. Researchers were able to discover the legitimacy of this syndrome by investigating this supposed superstition. In 1943, Lisgar Browne Eckhardt suggested that this phenomenon occurred due to crosstalk between two anatomically proximate cranial nerves: the optic nerve and the trigeminal nerve. The optic nerve is responsible for the pupillary response to light conditions. For instance, the pupil becomes smaller (constricts) in response to bright light, while it becomes bigger (dilates) in low-light conditions. Researchers found that the photic sneeze reflex could be elicited from subjects that had been in low-light conditions and were suddenly presented with a bright light.

It has also been demonstrated that photic sneezers do not have this response when their eyes are closed or covered. The running theory, thanks to Eckhardt and colleagues, is that when the optic nerve is activated in response to bright light, resulting in pupil constriction, the trigeminal nerve is also activated due to its proximity. This cranial nerve provides a sensory input to many areas of the face, including the nasal cavity. The trigeminal nerve picks up some activity from the optic nerve and interprets this as an irritant in the nasal cavity, resulting in a sneeze.

Can the photic sneeze response be hazardous? Not usually, but it could be cause for some caution while driving, especially when emerging from a dark tunnel into direct sunlight. Fortunately, your cranial nerves have another hack that can stop this sneeze response in its tracks. By pressing a finger between the nose and upper lip, one can take advantage of what is known as the philtral pressure technique to stifle a sneeze. When pressure is put on this part of the face, it activates some sensory neurons, and the activation of these neurons appears to override the trigeminal nerve response, preventing that annoying sneezing fit. 

Therefore, in the same manner that knowing some cranial nerve science can help bring about a sneeze when needed, it can also explain how to prevent sneezing when it would be awkward, such as in a quiet meeting or while taking a test. Sometimes a little bit of neuroscience goes a long way; it can even improve your memory.