You’ve probably heard that the eyes are the windows to the soul. Now, science reveals they are also linked to how we breathe. For years, researchers have studied pupil size to understand attention, emotions, and medical conditions. Now, a groundbreaking study has found that pupils change size in sync with breathing.
Pupils constantly adjust in response to light, emotions, and cognitive effort. Like a camera lens, they control how much light enters the eye. Shine a light into your eyes, and they shrink. In dim light, they expand. This process affects vision clarity—larger pupils help detect faint objects, while smaller pupils enhance sharpness.
Doctors rely on this reflex to assess brain function. A pupil that doesn’t react to light could signal a stroke or other medical emergency.
THE FOURTH PUPIL RESPONSE: BREATHING
Until now, scientists recognized three main pupil responses: to light, focus, and emotional or cognitive effort. A new study by researchers at the Karolinska Institute in Sweden and the University of Groningen in the Netherlands has confirmed a fourth response. That response is breathing.
Dubbed the “pupillary respiratory phase response,” this phenomenon shows that pupil size changes with breath cycles. Pupils are largest during exhalation and smallest at the start of inhalation. Unlike other pupil responses, this one originates within the body and occurs continuously.
50 YEARS OF CLUES
Hints of a connection between breathing and pupil size have existed for over 50 years. However, past studies lacked conclusive evidence. With pupil size widely used in medical research, confirming this link was crucial.
Through five experiments with over 200 participants, researchers recorded pupil size and breathing patterns. Regardless of lighting, task difficulty, or focus distance, the effect remained constant.
Participants were also asked to breathe through their nose or mouth, speed up, or slow down their breathing. In every scenario, the pattern remained the same: pupils were smallest at inhalation and largest at exhalation.
A NEW LINK BETWEEN VISION AND BREATHING
This discovery reshapes how we view both breathing and vision. It suggests a deeper connection between breathing and the nervous system. The next big question is whether these small changes in pupil size affect perception.
Though fluctuations are only fractions of a millimeter, they are comparable to pupil responses to cognitive effort. Scientists suspect this might subtly shift vision within a breath. It optimizes for detecting faint objects during exhalation. It also improves detail recognition during inhalation.
COULD THIS AID IN MEDICAL DIAGNOSIS?
Just as pupil reactions to light serve as a diagnostic tool, disruptions in the link between pupil size and breathing could indicate neurological disorders. These disruptions might signal such disorders. Future research could explore whether conditions like Parkinson’s or Alzheimer’s affect this newly discovered response.
This research is part of a broader effort to understand how bodily rhythms influence perception. Scientists now recognize that the brain doesn’t process external information in isolation—it integrates signals from the body as well.
For example, heart rate and digestion rhythms may also influence sensory perception. If breathing affects pupil size, could it also shape how we see the world around us?
This breakthrough opens the door to new studies on how internal bodily rhythms shape perception—one breath at a time.
