I think this assumption, though perhaps reasonable, is incorrect, for at least two reasons:
- 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.
It is well established that much “near work” during childhood (and even adults) contributes to myopia. Some populations are particularly vulnerable. Myopia also has a reasonable strong genetic component in humans and mice.
Classic example of gene-by-environmental interaction.
Maybe a darker environment makes it harder to see distant things, and the benefit of a brighter environment is to enable focusing on distant objects, which is presumably possible even for the poor souls confined to a small cage?
From what I have gathered, the leading hypothesis is that bright light induces dopamine production, and dopamine regulates the development of the eye. When these dopamine levels are too low, the eyeball elongates and becomes myopic.
- 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.
https://onlinelibrary.wiley.com/doi/abs/10.1111/opo.12207