According to Naked Scientist Chris Smith at least 20% of the population suffers from what is called the photic sneeze reflex - "sun sneezing".
While there is no exact explanation for it, Smith says the original theory is that this occurs because when you look at bright lights, it makes your eyes water, the watering makes your nose run and that tickles your nose and makes you sneeze.
He says it can be genetic.
The alternative explanation that we are comfortable with is that when light shines into the eye, the same circuits that are involved in making your pupil change size - that circuitry, some of it spills over in the brain stem into adjacent circuits which are involved in sneeze reflexes.— Chris Smith, Naked Scientist
In people that have that miswiring, when they look at a bright light the activity facilitates or increases the activity temporarily in the sneezing circuits in the back of your brain stem and that then moves you closer in the threshold for having a sneeze.— Chris Smith, Naked Scientist
About one person in five has this.— Chris Smith, Naked Scientist
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