Science Questions

Why do I sneeze on sunny days?

Tue, 3rd Apr 2012

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Question

Paul asked:

Something that's always intrigued me on sunny days when I'm inside the house, I'm fine, I go outside and I start sneezing. Now I don't know anybody else that this happens to, so do you have an explanation please.

Answer

Chris -   I think you've got what's called the photic sneeze reflex.  Photic as in light; Sneeze as in, well, sneeze!  This is apparently genetic, it seems to run in families.  Do you have any family members who do the same thing?

Paul -   I don't actually, no.  I've got family members, but I'm the only one that does it!

Chris -   Okay.  It affects about 1 person in 5 and what scientists think is going on is that there's a little bit of miswiring in the brain stem, which is the thing that connects the spinal cord to the main part of the brain which is where all of the circuitry is to control the size of your pupils and blinking, and that kind of thing.  So when a bright light shines into your eye, neural circuits get activated to cause a blink and your pupil to shrink – pupilloconstriction.  Some of this nerve activity also spills over into the parts of the nervous system that detect irritation in your nose and they trigger a sneeze.

People used to think it was because sunlight made your eyes water and that the tears running down into your nose tickled your nose a little bit, and that triggered the sneeze.  But when they did careful studies and in fact, the military have been researching this because what you don't want is someone who’s flying a supersonic jet to suddenly develop the photic sneeze response when they're flying into the sun and then have a fit of sneezing at the flight controls doing a thousand miles an hour!  So they were looking into this and they ruled out the tears tickling the nose, and they think it’s actually this miswiring and it does appear to be as I say, familial.  So, you get some people who have it sporadically and some who say, “Yup!  Lots of people in my family have this.”  So you have the photic sneeze reflex.

It happens to me when I get up in the morning in the winter time and I go into my shower where the lights are very bright, and you turn the bright lights on. All of a sudden you go from dark to very bright and I have this sneezing fit.

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