Have you ever noticed that sometimes you can see details with crystal clarity, while other moments your vision seems optimized for spotting movement or faint objects in dim light?

As it turns out, this shift might be happening with every breath you take.

For over a century, scientists believed that only three factors controlled pupil size: light levels, focus distance, and cognitive processes like emotions or mental effort. But groundbreaking research from Sweden’s Karolinska Institutet has uncovered a fourth fundamental factor that’s been hiding in plain sight: your breathing rhythm.