The size of your pupil systematically fluctuates with your breathing, according to a new study from Karolinksa Institutet. The pupillary respiratory-phase response (PRP response) means pupils are smallest at the start of each inhale and largest during exhale.

Traditionally, pupil size has been linked to light exposure, fixating on objects and emotional or cognitive states. This study, published in the Journal of Physiology, tested various breathing conditions, lighting environments and even participants without a sense of smell.

The consistency of their findings suggests that brainstem circuits drive this PRP response, independent of external influences.

Beyond deepening our understanding of vision and neural activity, these findings could have implications for human-computer interaction and clinical research, where pupil dynamics are often used as markers of brain function.

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