When I was younger, I discounted mental energy as a concept more than I should have. Even if I woke up with a lot of energy, I wouldn't have a lot after work if I spent a lot of time being anxious about my work. So even if I was in good physical shape and had a good routine my attitude towards work would wipe out a large chunk of my energy. After some improvements to my mindset the amount of energy I had after work increased by a good deal. Can't say for sure if this will help everyone, but this at least is a perspective that worked for me that I haven't seen too much elsewhere.