Gather in your resources, rally all your faculties, marshal all your energies, focus all your capacities upon mastery of at least one field of endeavor.JOHN HAGGAI
The raw material of personal performance and productivity is contained in your physical, mental, and emotional energies. Your body is like a machine that uses food, water, and rest to generate energy that you then use to accomplish important tasks in your life and work. When you are fully rested, for example, you can get two times, three times, and five times as much done as when you are tired or burned out.
One of the most important requirements for being happy and productive is for you to guard and nurture your energy levels at all times.