Have you left a dark movie theater on a sunny day and fallen into an unexpected fit of sneezes? If so, you’re not alone. A surprisingly large number of people have a strange reflex reaction where a sudden increase in light intensity causes them to sneeze.

It’s actually a recognized condition known as the photic sneeze reflex (PSR) or – not joking – ACHOO syndrome (Autosomal Dominant Compelling Helio-Ophthalmic Outburst).

It affects up to 33 percent of the US population, with 67 percent of those being female and 94 percent being from Caucasian backgrounds, according to a study from 1995 by the University of Alabama at Birmingham. Evidence also says the trait is partly genetic, so you can even pick it up on a 23andMe genetic test kit.

So, what’s it all about?

It’s a question that even perplexed the ancient Greek philosopher Aristotle some 2,300 years ago, when he posed the great question: “Why does the heat of the Sun provoke sneezing?” 

The truth is, there is little in the way of hard scientific studies on the subject, since it’s often seen as a humorous quirk rather than a debilitating illness.

Nevertheless, the leading theory says that a case of the solar sneezes might be caused by a minor glitch in the trigeminal nerve. One of the largest cranial nerves, it carries sensory information down separate branches across the face under the eye, the nose, mouth, and jaw. A handful of sneezing abnormalities are caused by stimulation to the trigeminal nerve terminal in the nasal cavity's mucous-making membrane.

People with PSR might have a bit of "cross-wiring" in this area's nerve network, causing inadvertent signals to be sparked across when they shouldn't be. The theory goes that bright light will cause the pupils in one's eyes to constrict. This nerve signal "accidentally" excites the nerve pathway, which prompts a flow of mucous a few centimeters away in the nose. This could be mistaken by the brain as an irritant and hence leads to a sneeze.

Some have gone even further to suggest it could be an evolutionary relic from our cavemen ancestors. When living in a musty and dusty cave with a smoky fire burning, a sneeze when heading outside could be advantageous as it clears the airways from pathogens and foreign bodies. However, that's purely speculation.

The hard truth still remains unknown. But after finding a perpetual source of energy and curing all known diseases, those scientists better get to solving ACHOO syndrome.