Fatigue is defined by a lack of energy or a sensation of tiredness that may improve with rest. It is a prominent symptom in numerous disorders. Many individuals with cardiovascular, endocrine, psychiatric (anxiety and depression), and neurologic (multiple sclerosis) disorders experience significant, sometimes debilitating, fatigue. Fatigue can also factor prominently in some sleep disorders. A thorough sleep history, including sleep quality, sleep quantity, work schedule, and perceptions regarding sleep, is crucial for the evaluation of fatigue. Identifying and managing underlying sleep disorders may improve sleep and attenuate fatigue.