THE ciliary muscle is like a circle around the lens in the eye, and it changes its shape to help us focus.
When the muscle contracts, it makes the lens fatter to see things close up; when it relaxes, it makes the lens thinner for distance.
The lens starts to get less flexible from your mid-40s, making the job of the ciliary muscle harder and its impact less effective. That’s when you need reading glasses.
Too much close reading (especially on tiny screens) can also cause the ciliary muscle to spasm, making distance vision blurry.
To keep the ciliary muscle healthy, Clare Roberts, a consultant ophthalmologist at Moorfields Eye Hospital in London, recommends limiting screen time and taking regular brief rests from sustained close work by looking at something far away every 20 minutes.
"We humans weren’t designed to look at very close objects for long periods,’ she says. ‘Remember to relax — the more you worry the more blurry your vision will get.’