> It's the spending time indoors that causes it apparently.

Not quite. It’s the lack of spending time outdoors.

Mixed together with (sometimes purposely) neglect of body and lots of reading small text with bad light conditions.

actually it's only the light part. recent studies from South Korea and Australia find, that lack of light exposure causes vision impairment

Not only that. It really helps your eyes to look at other things and more importantly, at things at different distance.

Plenty of studies for that, too.

(not strictly for "outside" though, you can also look out of the window regulary, it is so that your eyes refocus and relax )

Do you have a source? Everything I've read gave me the impression that we only have strong evidence that outdoor time is a factor, and the light exposure explanation is just a hypothesis.