I have problems working hard

Well when it comes to fizical work i dont have any problems working 12 hours per day or more ,but when working online its very dificult to work hard .Firstly if i stay too much in the front of the computer i get head pain and its very stresfull .So lets say i will work 5 or 6 hours per day then i will feel bad and my head take to explode .
Do you have any solution to fix that ? Thanks

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