From an optical perspective, focusing on something a few meters away is basically infinity for the eye. Even indoors you do that a lot, which makes the "focus close -> loose infinity" theory less plausible compared to light levels, which we know have a very significant impact on the whole body. Focusing very closely all the time (again, optically speaking, a screen 1 meter away is closer to infinitely far away than to a smartphone in the face) might do and explain something.