Too much close work. Nowadays, we spend much more time reading in close distance than our ancestors. Computers, but also smartphones and tablets cause our visions to be constantly focused at a distance of 20 to 60 cm. This constantly activates the process of so-called accommodation: we make an effort to focus in the vicinity. This stimulates our brain to make the eye short-sighted, because myopia means that you can see close up without any effort.
