Fatigue is a feeling of constant tiredness that can be physical, mental, or a combination of both. It is a symptom, not a condition, and can have many causes. The most common cause of fatigue is sleep deprivation, which can have negative effects on the body, such as impaired memory, decreased activity, and increased stress levels. Clinicians should ask patients about their sleep patterns and address any sleep issues, promoting sleep hygiene and addressing conditions like sleep apnea. Other potential causes of fatigue include heart disease, infections, hormonal imbalances, traumatic brain injuries, and depression. The speaker emphasizes the importance of identifying and treating the underlying cause of fatigue.