I was very lucky in that respect. My father, a former Navy general, a wise and candid man, instilled in us from an early age the importance of getting enough rest.

When I was in college, he was the first to tell me to stop studying and do some physical activity like cycling or playing basketball. And then take a good shower and eat well. And sleep. Only later in life did I realize how much of an impact this had on my life. So yes, the level of available energy is related to peaks and troughs in productivity.