Tuesday, April 13, 2010

Why does sunlight make you sneeze?

@sarahnb1 just asked this question on twitter, and I was interested because it links up with my previous post on sneezing and my interest in astronomy (vaguely...).

I've just had a quick look around the interweb, and apparently this phenomenon only affects a certain percentage (different sources suggest somewhere between 18% and 35%) of the population, of which I am one! As is my dad and, I'm assuming, @sarahnb1.

It's known in scientific circles as the photic sneeze reflex which has a certain ring to it, I think. It has been noticed and studied since at least Aristotle, but no conclusive scientific explanation has been decided upon. Historical hypotheses include the reflex being caused by heat (refuted by Francis Bacon who attempted to recreate the situation with his eyes closed*) and moisture (the idea being that the sunlight made the eyes water, which leaked into the nasal passages**).

The most commonly accepted partial explanation appears to be a certain cross-wiring in the parts of the brain responsible for processing input from the ocular and nasal senses. Put simply, a sudden inrush of light information from the eyes is misunderstood by the brain and taken as information being sent from the nerve endings in the nasal passages. This means that the brain thinks something unusual is up the nose, and orders a sneeze as a result.

The condition is not seen to be harmful in any way, and no studies have shown it to be linked to any other more serious illnesses or complaints, which may go some way to explaining why it has gone so long without a proper explanation. Some sources I came across suggest that some scientists are hoping that studies of the reflex may provide insights into how the brain processes light signals which are relevant to learning about conditions such as epilepsy and migraine, both of which are known to have light-sensitive triggers and effects.

Here's a video of a kid photic-sneezing. It goes without saying, I'm sure, but please, please, please do not look directly at the sun, even if you're trying to find out if you're a photic sneezer. Sensitivity evidently varies with this one- some 'sufferers' say that they can't go outside on a sunny day without sunglasses because they just can't stop sneezing. Personally, it happens to me when I move from a darkness into bright light, such as leaving the cinema on a sunny day; once I get used to the new light levels, the sneezing stops.

* All conditions were the same apart from the presence of light; the sneeze did not manifest where previously it had.
** More recent experimentation has concluded that the sneeze comes about far too quickly for this to be the case.