Every day starts with hope. No matter what habits you are trying to practice—better eating, more exercise, more focus on things that matter—the morning is so full of promise. But as the day goes on, it is harder to maintain those good habits.
It is just like running. You start off at a solid pace, but as you run further, you get tired and slow down. It takes effort to maintain a pace throughout a run. It takes effort to maintain good habits throughout the day.