Because of the limited energy that you have, you…
Because of the limited energy that you have, you do not want to spend your energy on non-productive matters. So make sure that whenever you are expending energy you are getting something of value in return.
— from Fuel (Energy)
In the book
Protect the reserve, and start at dawn. The first real choice of the day is whether to rise eagerly and take the controls, or lie there and let the day choose for you. From there, guard the fuel: refuse to spend it on the non-productive, learn to say no, and keep not only a to-do list but a "to-don't" list of what you will no longer indulge. Make the good habits easy and the draining ones inconvenient so the structure spares your willpower, and settle conservation, storage, and wise expenditure into habits rather than daily decisions. — Fuel (Energy)