medical symptom usually of physical and/or mental weakness that does not resolve with rest or sleep

Fatigue or tiredness is a physical or mental feeling. It happens when someone is ready to sleep or has done a lot of physical exercise. The person may feel weak, slow or sleepy. They may yawn and may find it difficult to keep the eyes open. Usual fatigue goes away when you rest or sleep.

This man is showing fatigue, as his arms are folded and his head is resting on them.

Fatigue may also be caused by physical or mental illness, jet lag, drug or alcohol use, medications, blood loss, malnutrition, starvation, stress or boredom.