Maybe you would rather relax in your hammock, or watch television, or surf the net, or whatever it is that you like to do, than sit behind that computer and squeeze out code or fill in parameters in your programming environment.
Then when you really get to sit behind your computer, still your mind is traveling. Physically you are at the job site but mentally you are absent.
Well, after all, you will have to get down to earth, concentrate, and do the real work.
Stop postponing. Get over your internal resistance, over your fatigue, your depression, over your laziness, whatever it is that keeps you from being productive.