I think (weakly) the nearsightedness issue is indeed from not exercising focusing rather than sunlight.

But apparently there is an issue where mothers with low vitamin D in Scandinavia (especially immigrants from sunnier places) are more likely to have autistic children.

Anecdote, I started fidgeting by varying eye focus and depth - can't get forced to sit still if your fidgeting is barely perceptible - at a very young age and have the best range of accomodation of anyone I know. Can also easily perceive chromatic aberration/my eyes needing a different focus setting for different wavelengths.

I'd love to know if some sort of focus training exercises for eyes starting as children would help everyone.

Or maybe the effect takes years of sustained lack of light to work, and northern summers compensate?