Diet, rest, exercise are keys to wellness. These are under our personal control. No one holds a gun to your head forcing you to eat junk food loaded with sugar, salt and fats. The use of alcohol and drugs is an act of personal choice. If you choose to smoke cigarettes, the physical consequences are a reduction of overall wellness and energy.
Mental energy is inexhaustible.
The brain is absolutely tireless. Even when you are asleep, your brain is busily at work on your behalf. What you interpret as fatigue comes from your body, never your brain. The brain extracts the best of the total person. It gets first shot at the blood's oxygen and the body's heat.