Why do you sneeze when you walk into bright sunlight after you have been inside for a while?

Question: Why do you sneeze when you walk into bright sunlight after you have been inside for a while?
- Keywords:
  biology,
  health,
  human
Asked by radsare1005 to Daniela, Hannah, Ian, Jono, Mark on 25 Jun 2013.
- Hannah Brotherton answered on 25 Jun 2013:
  
  Hi radsare1005,
  This is freakkky, my friend asked me the same question yesterday lol.
  So it is called the photic sneeze reflex is caused by how strong the light is. That is why we only sneeze when we look at the strong light of the sun. No one really knows why we have it, but this is what I told my friend yesterday.
  
  We have reflexes in our body that protect us from harm, like the reflex that pulls our hand away from a hot plate. So this reflex that makes us sneeze must be protecting us from something….what is this something??
  
  The sun can really damage our eyes and if we look at it directly we can feel our eyes burn and it becomes painful, But I think sometimes, we can look at the sun too directly without knowing it and our brain makes us sneeze to quickly stop us from looking at the sun. It is protecting us.
  
  So people take antihistamines, which stop hayfever, but they also take it to stop them sneezing when they look at the sun. Because in some people this can get really annoying. But I don’t think they should take them, because it must be protecting us from the damaging rays of the sun.
  
  But some people can have it really bad when they are driving, so I definitely think they should take anything they can to stop them sneezing from the sun.
  
  Btw I told me friend this, and now she thinks she is an expert in sun sneezing lol
  
  😀
- Mark Hodson answered on 25 Jun 2013:
  
  Hi radsare1005,
  
  no idea, I always thought it was one of those things but what Hannah says makes perfect sense. We’re all sun sneeze experts now!
  
  cheers
  
  m
- Ian Wilson answered on 25 Jun 2013:
  
  Hi radsare1005,
  
  This is a really interesting one – apparently this only happens to about 1 in 3 people. It’s called the ‘photic sneeze reflex’. ‘Photic’ means ‘relating to light’, so it literally means a sneeze in response to light – imaginative name, huh?
  
  Nobody knows for sure what causes it but it’s suspected that it’s to do with the brain getting confuzzled! Normally, you sneeze because there’s something irritating a nerve in your nose called the trigeminal nerve, like dust. Now, this nerve normally works separately from nerves involved in any other senses.
  
  But, when you walk into bright sunlight, the optic nerve in your eye (which responds to light) receives an overload of information as loads of light enters the eye. Some of the signal that should be sent by the optic nerve is passed onto the trigeminal nerve, instead. This triggers a sneeze.
  
  You can test this theory yourself. If you’re a photic sneezer then close your eyes when you walk from a dark building into bright light. You won’t sneeze, because your optic nerve isn’t receiving that overload of information so your trigeminal nerve isn’t being given any spare signal. Pretty neat experiment on a sunny day!
  
  Hope that helps,
  
  Ian
- Jono Bone answered on 26 Jun 2013:
  
  Hey radsare1005,
  
  This doesn’t happen to everyone, it is determined by your genetics and only affects 18–35% of people.