Ciliary muscle

The small ring of smooth muscle inside the eye that contracts to change the lens shape for near focus and relaxes for distance vision.

The ciliary muscle drives accommodation — the eye's ability to switch focus between near and far targets. Sustained near focus (reading, screens, detail work) keeps the muscle contracted; over 20 or more continuous minutes, this produces the burning, fatigued feeling at the heart of digital eye strain. Looking at something at least 20 feet away allows the muscle to relax, which is the mechanism behind the 20-20-20 rule. Age-related stiffness of the ciliary apparatus is the root cause of presbyopia.