Lack of discipline though is why I'm currently posting =P.
It's definitely a muscle, I also have a theory of personal inertia. I find that the more I need to get done, the more I'm able to do. Newton's first law states that an object in motion tends to remain in motion, and an object as rest will remain at rest. The more you keep yourself in motion and productive, the easier it will be to maintain that. Setting yourself up to get some easy production out of the way early is a great way to get your day started properly and maintain that productivity all day.