Sneezing in response to naturalistic bright light exposure [version 2; peer review: 1 approved, 2 approved with reservations]

Background The photic sneeze reflex (PSR) is a common but underexplored phenomenon where bright light triggers sneezing, affecting ~30% of the population. This study aimed to characterise the light conditions inducing PSR. Methods One male PSR-affected participant logged sneezing events during a 30-day real-world light exposure study. An indoor setup using a multi-primary LED source and an integrating sphere delivered 30-second light stimuli while pupillometric data were collected. Results A total of 82 sneezing events were recorded, averaging 2.73 sneezes/day (range: 1–6 per event). Illuminance increased tenfold before sneezing, peaking 2 minutes prior, and returned to baseline within 10 minutes. Despite exposure to 150+ stimuli, artificial sneezing was not induced, though high illuminance consistently triggered tickling sensations. Conclusions Sudden increases in environmental lighting can provoke PSR. While artificial stimuli elicited only tickling, further refinement of the protocol could enable reliable PSR induction, thereby facilitating mechanistic research.