Moods are adaptive, subjective feelings created by brain neural networks. Vigor is a positive mood described by feelings of energy and enthusiasm. Fatigue is a mood state that is moderately and negatively related to vigor, yet can be conceptualized as separate from vigor. A fatigued mood state is characterized by feelings of exhaustion and weariness. Although only about 1% of the population suffers from the severe psychiatric disorder known as chronic fatigue syndrome, persistent and debilitating feelings of low energy and fatigue occur in as many as 25% of people in community samples. The prevalence of persistent fatigue is approximately 50% among groups of people suffering from major medical conditions, including cancer and heart disease. Because physical inactivity contributes to feelings of low energy and ...