Upon waking there is a moment of fuzziness, and you know for certain that no, you do not want to get up right at the moment, and really you'd rather roll over and go back to sleep. After all, it is Sunday and there's nothing pressing to do right at the moment. But then your bladder starts sending messages of urgency to your brain, and muscles that you mercilessly beat in a workout the previous day begin squealing in agony and seize up. Then your mouth and teeth say 'clean us, for we are filthy' and in a minute you are up and stumbling towards the bathroom.
Once all emergencies are attended to and tasks completed, you discover that you are still sleepy. So you go back to bed and curl up and try to go back to sleep. But then the dog starts walking around and clicking his toenails on the bare wood floor, and the horses start whinneying off in the distance, and you know for certain that you are not going to be able to sleep in this morning.
So you get up, and get dressed, and prepare yourself for the day ahead, and lament the fact that you couldn't go back to sleep.