I assumed outdoor time helps because your eyes spend time focusing at a greater distance. Going outside, but only to use a phone/ereader/book is likely not effective.
- There are animal model studies which vary environment brightness and show a causal relationship between darker environments and myopia. The animals in the darker experimental groups aren't reading etc, and the animals in the brighter experimental groups aren't in larger cages.
- There are studies on myopia that varied indoor lighting brightness and showed brighter lighting later/decreased myopia onset. Classroom sizes (or distances from desk to blackboard, etc) did not change. These studies also found a bunch of other variables were important including longer sleep time, and less screen time.
Classic example of gene-by-environmental interaction.
Here is a recent reference and meta-analysis:
The leading hypotheses a few years back is that the outdoors is very bright. Brightness levels that cannot be easily reached indoors.
surely when we squint from bright light we are straining the eye. The idea, then, is that our development expects some amount of eye strain growing up.