You know the feeling. You just got home from work. It's been a long, hard day, and you collapse on the sofa. You don't have the energy for anything. You know you have to cook dinner, to do your washing, but you can't. You can't even begin to think about doing them. Because you're completely out of energy.

You're sitting there, trying to block out your thoughts, trying to focus on the tv. But your partner comes over and asks you how your day was. You snap at them. You don't mean to. But replying is impossible. Your brain just wants to shut down. You don't have the energy.

When you start looking at life from this perspective—that you have a mental energy limit you can't push beyond—things start making a lot more sense.