What is the Cause of Nearsightedness?

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Nearsightedness, or myopia, is discussed in terms of its causes related to the eye's lens and the surrounding muscles. One explanation suggests that nearsightedness arises from the lens becoming permanently curved, losing its ability to flex back to a flat state due to prolonged close-up focus, similar to a spring that has lost its elasticity. Alternatively, another theory posits that the eye muscles responsible for adjusting the lens are locked in a contracted state, preventing relaxation and thereby keeping the lens in a flexed position, which restricts distance vision. The discussion raises three potential explanations for nearsightedness: the lens losing elasticity, the muscles being unable to relax, or a combination of both factors. The complexities of myopia highlight the interplay between the lens's physical properties and the functionality of the eye muscles.
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I have become nearsighted after too much reading/computer staring. And obviously it is because my eyes are spending most time focused at <=80cm distance.

Now my physics teacher said when in optics that nearsightedness was caused by the lens being permacurved, so that it could not flex back to it's flat configuration due to being held curved for so long. Somewhat like a spring that has lost it's ability to flex back due to some chemistry reasons.

This is fully logical, nothing wrong with that but then I read another explanation online;

Nearsightedness is caused by the muscles being locked in place, so that they cannot "relax" to let the lens is restricted in a flexed state by the eye muscles. But if the muscles were to relax the lens would automatically go to it's normal state and you would focus on the distance.

Now my question is, which of the following is correct:

i) nearsightedness is caused by the lens having lost it's elasticity and ability to flex to a long distance focus point when no force acts upon it.

ii) Nearsightedness is caused by the focus muscles that act on the lens being permanently locked and unable to relax to allow the lens to relax itself.

iii) Nearsightedness is or can be caused by ii), i) or both ii) and i) at the same time.

I excuse my lack of biological terms, STEM guy here.
 
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