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 time, 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.
From the book “Eat That Frog!” by Brian Tracy