Fatigue of the voice is inability to continue to phonate for extended periods without change in vocal quality. The voice may fatigue by becoming hoarse, losing range, changing timbre, breaking into different registers or by other uncontrolled behavior. These problems are especially apparent in actors and singers. A well-trained singer should be able to sing for several hours without developing vocal fatigue. Fatigue is often caused by misuse of abdominal and neck musculature, or over-use (singing or speaking too loudly, too long). Vocal fatigue may be a sign of general tiredness, or serious illnesses such as myasthenia gravis.