Brilliantly right – fatigue is the enemy:

In fact, it’s not the number of hours we work that determines the value we create. Rather, it’s the quality of energy; we bring to the hours we work. By renewing regularly, it’s possible to get more done, in less time, at a higher level of quality, more sustainably. When we’re less fatigued, we’re not only less prey to negative emotions, we’re also more likely to access the positive ones we need to feel to perform at our best.