I've never suffer RSI or SAD. I don't know what is the reason, because, I don't do some exercise.
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I think I sit correctly during my work. However, sometime, my nape aches and my eyes are very tired.
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You should have a good screen: The screen should be bright and sharp, and it should not have flickering.
There should be any reflected.
You should clean the screen: The dust does no clear screen.
You should work in a room with good lighting.
You should use a legible font size.
You should take breaks and practice visual exercises.
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I think that exercises are very good. I've never do them before, but now I'll try to do them.