Fatigue is the result of emptying oneself out for the cause of Christ. An empty calendar and well rested body is God's reward for mediocrity. Or put another way by J. Oswald Sanders says, "Mediocrity is the result of never getting tired."
Doing something great for God is going to cost you something. Listen, I'm not advocating that we all have to run at breakneck speed to accomplish something with our lives. Sure, we need to organize our schedules for times of refreshing and renewal as well as allowing for opportunities for late nights and early mornings.
However, everything in life boils down to a lot of prayer and a lot of hard work. Yes, fatigue is a sign that I need some rest but it's also a sign that I'm giving my all for Christ. So, I'm going to work as if it all depends on me and pray as if it all depends on God.