stop spending too much time on the computer during the evening

...sat at the computer during the evening. Sometimes I will just sit lookign over the same things time and time again or I will sit doing next to nothing and before I know it I have spent hours at at the computer.
Sometimes I will sit at the computer when I can’t think of anything to do or if I am bored but I must try and find more things to do even if it’s studying or working because sitting at the computer for so long can’t be healthy plus there is so much more things in life I can be doing.
I do a lot of work at the computer for my job but somedays there aren’t as much things to do and yet I will still sit at the computer when maybe I could be outside getting some fresh air or doing some exercise or even housework so I need to motivate myself into stopping myself from sitting down at the computer and doing nothing of interest.