Fatigue is a feeling of excessive tiredness or lethargy, with a desire to rest, perhaps to sleep. Often it causes yawning.

Chronic Fatigue Syndrome (CFS), which is also sometimes referred to as Chronic Fatigue Immune Dysfunction Syndrome (CFIDS) or Myalgic Encephalomyelitis (M.E.), is a condition characterized by the primary symptom of a prolonged, persistent debilitating fatigue not relieved by rest and not directly caused by other conditions.