Okay, so I haven't published anything on this, but I have asked several optometrists explicitly. They have all said that the worst that can happen is you get a headache from eyestrain the first few times you use it, that you might feel tired, but ultimately it goes away, there is no chance for permanent damage. Their view was that the muscles tire out, that you can't strain them to the point of injury, and it just becomes uncomfortable and unworkable, but not damaging.