I live with a chronic condition that is slowing my metabolism and I don't think that metaphor is really apt. If it were an energy budget I could cram it in a few hours, or doing only a few things and feel well all the day. This is not how it works, it's more like a hard cap on the number of hours a day I can do things, and a cap in the intensity as well. What I noticed recently is that it creates a very effective negative feedback loop: since keeping things going on is taking all my energy already, I have no energy to consult with would improve the situation.