A quote by Aristotle (ancient Greek who pretty much founded modern science, if you're wondering): 'Why is it that one sneezes more after one has looked at the sun? Is it because the sun engenders heat and so causes movement, just as does tickling the nose with a feather? For both have the same effect; by setting up movement they cause heat and create breath more quickly from the moisture; and it is the escape of this breath which causes sneezing.'
If you have never suffered from sneezing fits you will have no idea what I am talking about and will probably think I'm crazy. But if you are one of the 20-30% of the population that suffers from Autosomal Dominant Compelling Helio-Ophthalmic Outburst Syndrome (shortened to ACHOO in an incredible burst of humor by those responsible for the name) then keep reading.
To put the last sentence into English, the autosomal bit means it is inherited, dominant means that you only need one copy of the dodgy gene to get it (rather than two for a recessive trait), compelling means you get no choice in it, Helio-Ophthalmic means whenever you see bright lights and outburst means, well, an outburst. This is the scientific name for a condition where you sneeze uncontrollably when you see bright lights, like the sun.
When a bright light stimulates the optic nerve through the eye, it sometimes causes a reflex reaction in the nearby olfactory nerve, triggering a sneeze. It is a reflex action, like when you stick your hand on the iron and your body whips it away without you having to think about it. Why this happens though, no one seems to know. How sneezing when you look at a bright light is helpful remains undetermined. Perhaps it is an accidental crossing of eyes and nose nerves.
The syndrome and the number of sneezes appears to run in families, so if your mum sneezes 6 times whenever she goes outside, you have a 50-50 chance that you will as well! People who sneeze 43 times on exposure to the sun have been recorded so consider yourself lucky if you just sneeze once or twice! Sneezing fits can result from lots of weird stuff, like eating too much or brushing your hair too. Bless you