In a virtuous circle, research shows, being happy energizes you, and at the same time, having more energy makes it easier for you to engage in activities—like socializing and exercise—that boost happiness. Studies also show that when you feel energetic, your self-esteem rises. Feeling tired, on the other hand, makes everything seem arduous. An activity that you’d ordinarily find fun, like putting up holiday decorations, feels difficult, and a more demanding task, like learning a new software program, feels overwhelming.