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The photic sneeze is nothing to sneeze at! Even our beloved deep-thinker, Aristotle, pondered this phenomenon: "Why does the heat of the sun provoke sneezing?" And this, my friends, is the question that I present today. Do we know why a large number of people experience the photic sneeze, a sneeze reflex when someone is rapidly bathed in bright lights (esp. when emerging from the dark)?

Is something happening in our occipital lobe, such as the trigeminal nerve, in which the body's wires are essentially crossed? In other words, can visual stimulation really be a catalyst for a sneeze?

Or is something else going on here? Is it a nervous, startled reaction from primal times that had some sort of benefit?

Or are there typically more particles in sunlight that can trigger sneezes?

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    Have you also considered that in order to look up at the sun you must tilt your head back, and that this could be the primary cause of sneezing in that scenario?
    – Travis J
    May 4, 2017 at 22:36

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To answer this question as to why some 35% of people may experience the photic sneeze reflex requires knowledge of how the photic reflex occurs and its function, and both are currently unknown. It may be a neutral trait, or it may even have some evolutionary advantage.

It's known now that simulating a sneeze by passing fast moving air over nasal cilia increases the ciliary beat frequency and it has been hypothesized that this resets the nasal airways by clearing out debris and other particles. One could theorise that this might temporarily improve the sense of smell which might be advantageous in a hunter gatherer society.

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