There could be a simple answer to why you can't muster up the energy to complete tasks or why you've been having poor judgement — decision fatigue [1]. Simply put, making too many decisions can negatively affect your ability to make decisions later on. That's because decisions have a tendency to drain willpower [2].

Even President Barack Obama employs strategies to battle decision fatigue by minimizing the amount of more "frivolous" decisions, such as what to wear and to eat. He achieves this through certain strategies, like only wearing gray or blue suits and routinizing his day. "You need to focus your decision-making energy. You need to routinize yourself. You can't be going through the day distracted by trivia," Obama told Vanity Fair [3].

To lesson the mental fatigue, don't take too much time agonizing over decisions and try to routinize your day so you don't have to make a decision on whether to do tasks like going to the gym. And because decision fatigue clouds your judgement, make your more important decisions early in the day or hold off until the next morning. You can always make your less urgent decisions the night before as well. For example, you can decide what you will eat and wear at work the next day the night before.