Mental energy refers to our capacity to perform cognitive functions, such as concentrating, problem-solving, directing our attention appropriately, and deciding what to have for dinner (is this a daily challenge in anyone else’s house?). When it’s depleted, we may experience decision fatigue, have a hard time focusing, or feel overstimulated. As is the case with our physical energy, our mental energy reserves increase and decrease throughout the day. The key is to refuel when the tank is ¼ full instead of waiting until you hit empty.