Why does looking at bright light trigger sneezing in some people?

Are there any recent studies that have found a cause for this Photic sneeze reflex? The Wikipedia article only references studies pertaining to the effect, stating that the cause is unknown. The article also states that "the condition affects 18–35% of the population", which seems to be quite a large percentage.

What could have lead to the development and persistence of such a genetic trait in humans? Are there any evolutionary advantages to this?


2 Answers 2


You are talking about the photic sneeze reflex.

The mechanisms are not entirely understood, but it affects 18-35% of the population.

According to Wikipedia (although this passage is not sourced):

The probable cause is a congenital malfunction in nerve signals in the trigeminal nerve nuclei. The fifth cranial nerve, called the trigeminal nerve, is apparently responsible for sneezes. Research suggests that some people have an association between this nerve and the nerve that transmits visual impulses to the brain. Overstimulation of the optic nerve triggers the trigeminal nerve, and this causes the photic sneeze reflex.

And, it could be advantageous:

On the other hand, some people with the trait feel that it is advantageous. In the event that nasal discomfort occurs, but to an extent that is insufficient to induce a sneeze, intentionally seeking and finding a light source facilitates the sneezing process and is in turn a mode of relief.

Also, anecdotally, as I too photosneeze, I can see an advantage: should the atmosphere become weaker in the future, and allow more light to enter than we are accustomed to, it would discourage looking directly at the sun.

  • $\begingroup$ “not entirely understood” clashes with the explanation you give just below. The mechanism is entirely understood. $\endgroup$ May 26, 2011 at 14:11
  • $\begingroup$ @Konrad That is just one of many possible explanations. It is not known how the specific trait causes this, and why it persists to today. $\endgroup$
    – Thomas O
    May 26, 2011 at 14:48
  • $\begingroup$ This happens to me every day. Someone told me once that it's because the 'light rattles the nerve in your eye' $\endgroup$
    – rmx
    May 26, 2011 at 15:22
  • $\begingroup$ @Konrad: Just because a phenomenon is "not entirely understood" doesn't mean that people don't have any ideas. The reverse is also true; in fact, I can think of very few things that are entirely understood. $\endgroup$
    – voithos
    May 26, 2011 at 21:45

In high school biology, we had to read a book called "Survival of the Sickest". In this book, the idea is presented that this reflex evolved during humankind's "caveman" days. The author presented the anthropological induction that after spending periods of time in a dark, dank cave, mold spores begin to accumulate on the human body and inside immediate and open orifices. The nose, being moist, open, and pulling air in, would then be prime nesting ground for mold. Some claim, then, that humans evolved this reflex as a way of clearing the nose of mold and other impurities upon exiting a cave and entering the brightly lit outside world.

  • $\begingroup$ This trait might be correlated to other traits not necessarily be or ever been under selection. I think the first step would be to understand the mechanism behind this connection and whether people from different population have the same rate of appearance of this trait (e.g. in northern population should be more frequent)... $\endgroup$
    – have fun
    Aug 31, 2017 at 9:16
  • $\begingroup$ I agree, but the question was regarding why we evolved this trait, not how the body does it. And that's anthropology; we take the most likely and feasible conclusion from a sea of other likely conclusions. I just wanted to add a new dimension to the existing responses. You know, providing another path in which people can direct their research. Regarding correlating rates of expression and geolocation, I don't think northern populations would inherently show more expression, and I believe the author introduced some evidence of this type, but I cannot recall and I no longer have the book. $\endgroup$ Aug 31, 2017 at 9:32

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