A peculiar and unexpected reaction occurs for many when they step into bright light: an immediate bout of sneezing. This phenomenon, known as the photic sneeze reflex (or the “ACHOO syndrome”), affects an estimated 10% to 35% of people, hinting at something far more than a simple sneeze. It’s genetically driven and typically produces a consistent number of sneezes, often two or three, each time it’s triggered.

Researchers are still figuring out why this reflex happens. It appears to be linked to changes in light intensity, such as moving from a dimly lit room into bright sunlight, rather than being triggered by specific light wavelengths or flickering. Filtering lenses don’t prevent it, suggesting the reflex is hardwired into our system.

Though typically harmless, the photic sneeze reflex can be dangerous in certain situations, such as when emerging from a tunnel while driving or in the case of pilots transitioning from dim cockpit lighting to bright sunlight, where a sudden sneeze could seriously impair performance.

In essence, the photic sneeze reflex remains a biological mystery. What we do know is that it’s genetic, repeatable, and tied to shifts in brightness, not light color or flicker. Whether it’s a quirk of evolution or a curious neural crossover, the experience serves as a vivid reminder that our bodies still hold secrets waiting to be unraveled.