Nature ran an article The myopia boom last month, focusing in the explosive increases in cases of myopia in many countries (with some Asian countries going from ~20% in 1940 to ~80% in 2010).
The precise cause is not determined with certainty, but there is a very good correlation between developing myopia and spending time indoors:
Researchers have consistently documented a strong association between measures of education and the prevalence of myopia.
[...]
one in five of the children had developed myopia, and the only environmental factor that was strongly associated with risk was time spent outdoors
children who spent more time outside were not necessarily spending less time with books, screens and close work. “We had these children who were doing both activities at very high levels and they didn't become myopic,”
The proposed mechanism for this is the lack of light:
Retinal dopamine is normally produced on a diurnal cycle — ramping up during the day — and it tells the eye to switch from rod-based, nighttime vision to cone-based, daytime vision. Researchers now suspect that under dim (typically indoor) lighting, the cycle is disrupted, with consequences for eye growth. “If our system does not get a strong enough diurnal rhythm, things go out of control,” says Ashby, who is now at the University of Canberra. “The system starts to get a bit noisy and noisy means that it just grows in its own irregular fashion.”
But this is the subject of on-going research, and not everyone agrees. Other factors may also play a part:
Some researchers think that the data to support the link need to be more robust.
[..]
He says that the greater viewing distances outside could affect myopia progression, too. “Light is not the only factor, and making it the explanation is a gross over-simplification of a complex process
