Viewing Computer screen continuosly induces stress and headaches, remedy??!!

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I'm a software engineer and my job requires me to sit in front of the computer screen for at least 8 hrs a day, for a few days I've developed persisting headaches and heaviness. My head is heavy when I wake up in the morning, I've adjusted my monitor brightness and contrast levels as suggested by one of the websites. Do you also have similar problems,please share your experiences with me, and if possible the ways you used in order to minimize this.Is it better to sit perpendicular to the screen or at some angle?