Many of our readers have likely seen this reflex in action: a person walks from a dark area into the sunlight and sneezes. What's going on? Why does sudden brightness cause a sneeze? The earliest theories about the photic sneeze go back to the era of Aristotle, but the most widely accepted modern interpretation involves a small genetic quirk and an overstimulation of nerves in the skull.
Sneezing can be linked to a number of factors. Allergies, infections, irritation from harsh chemicals, a sudden change in temperature, or simply the need to clear our airways top the list. However, there is another rather strange reason: bright light.
The best way to test the relationship between bright lights and sneezing is to step out of a dark place and look at the sun. Most people won't feel anything, but between 18 and 35 percent will sneeze. Welcome to the “bright light sneeze reflex”, also known as “photic sneeze”.
Photic Sneeze: When the Sun Makes You Sneeze
One of the first to ask why the sun makes us sneeze was none other than Aristotle. His theory was that the heat of the sun made the inside of the nose sweat, and the sneeze released the excess moisture. An interesting viewpoint, but ultimately incorrect, because Sir Francis Bacon realized that closing one's eyes prevented the photic sneeze from happening.
As an alternative, he proposed an increase in eye moisture, which was partly transferred to the inside of the nose, triggering a sneeze. The problem is that the reaction occurs too quickly, so moisture accumulation cannot be the cause.
Several studies in the sixties indicated that the photic sneeze is genetically transmitted and only requires a single copy of the gene from one parent. The low percentage of people with the reflex minimizes the possibility of an evolutionary advantage, or its existence would be much more widespread. The responsible gene is located on the second chromosome, just a single letter in the DNA: those who sneeze have a C, while those who don't have a T.
This information came to light in 2010 thanks to an online study involving ten thousand participants, and subsequent genetic analysis of those with photic sneeze. This brings us to the trigeminal nerve. The ophthalmic nerve (V1) of the trigeminal nerve is overstimulated by bright lights, causing a kind of “jump” to the maxillary nerve (V2), which leads to a sneeze.
