When I look at the sun on a bright day it sometimes makes me sneeze. Why is this?

Zoe,

You have asked one of my all-time favourite bits of biology trivia and I am delighted to have got here first to answer it. However, the answer is rather complicted, so bear with me!

First off, your sneeze is what we call a 'reflex' action. It is an action that you cannot control (you can't make yourself sneeze, like you can stick your tongue out). When your nose is stimulated by something that irritates it (like pepper) the nerves send a message to the brain where the sneeze reflex set of nerves send a message to your lungs to make you sneeze and flush out the nose.

When nerve cells are stimulated they pass on a chemical and electrical message to the next nerve cell in the chain. However, if it is a very strong message, this might also leak out and stimulate nearby nerve cells. You can probably now see where this is going.....

So, when you look at the sun or a bright light, your eyes (and their nerves) suddenly have lots of very strong information to pass to the brain. So in addition to passing on their message, they also 'leak' a bit. Part of the path for the optice nerves (from your eyes) happens to be close to your sneeze reflex and so it can be triggered by accident.

So, when you look at the sun, you eyes accidently trigger a sneeze. I hope that's clear for you.

Last edited by David Hone (16th Feb 2007 14:27:18)

This happens to me too, but the worst time for me is an overcast but dry day. I have to wear sunglasses while driving or I'm sneezing so much I worry I might crash.

Apparently it's also a test for pilots in some military branches. They don't want people who can be severly incapacitated for a few seconds by something as mundane as the sun.

I'm going to play devil's advocate and say I don't believe that for an instant! The idea that the trigeminal nerve which controls the motor aspects of a sneeze is "close" to the optic nerve and thus somehow the bright light stimulation of the optic "spills over" to the trigeminal is to my mind just not tenable from a neurophysiological or neuroanatomical point of view! The trigeminal nerve is equally close to many other cranial nerves yet the response we are talking about is very specific. I think it would be better to say this is an intersting  phenomenon that is under-researched and the cause is unknown. What IS of interest is that it is clearly genetic - only about 25% of people exhibit this phenomenon.

I have to say, I'm with David on this one... I used to believe it, but after a brief but informative neuroscience module in second year, I'm not so sure.

What about people half way though... I find that I can look at bright lights fine, unless I'm half about to sneeze and then looking at one will set the sneeze off! (Which can be quite useful...) but that seems to be half way between what you're talking about and no reaction at all.

I'm also an occasional photic sneezer but I've never observed it happening in my children (which doesn't mean it has never happened!). I think there were some studies awhile ago that strongly suggested that a predisposition to photic sneezing is genetic, as David says. It will be interesting to see if the 'condition' is non-homogeneous, e.g., if there are adult-onset forms. I agree with David about the trigeminal* and I think there are some studies that have shown that the sneezing is still present in blind-folded people. Recent studies (e.g., http://www.ncbi.nlm.nih.gov/pubmed/20169159) suggest that visual cues are important!

* that is assuming the condition is not caused by a neuroanatomical change that brings trigeminal and optic nerve processing into close(r) functional proximity!

Last edited by Steve Lolait (26th Jul 2011 14:05:48)

Hello,

Like Adam, I'm personally at the halfway house of photic sneezing. But more importantly, I was taught that the maxillary branch of the trigeminal nerve (cranial nerve 5, part 2) is responsible for the nasal mucosal sensory input to the brain. This nerve branch is also sensory for the surface of the upper cheeks and for the skin over your nose.

Thus, when much sunlight tickles this area, in some people it may trigger the sneeze reflex, just as pepper does when inside the nose - simply because the sensory input is the same for both regions. This makes sense because photic sneezers often still sneeze when their eyes are closed - its the light on their face that may create a response. I heard that large or sudden changes in light intensity may trigger the reflex, but as David says, I'm sure there's much more to discover about this unusual trait.

Nick

Last edited by Nick Hayward (26th Jul 2011 17:39:55)

I'm sure this is no help, but I was told years ago that sneezing in bright sunlight was because photons get up your nose.