Fatigue is a common yet poorly understood symptom of cancer treatment. The document discusses several potential factors that can contribute to and maintain cancer-related fatigue, including biological, behavioral, thinking, emotional, and environmental factors. Biologically, changes in nerve function and immune factors may play a role. Behaviorally, all-or-nothing activity patterns and excessive rest can both perpetuate fatigue. Negative or unhelpful thinking styles may also influence fatigue levels. Emotions like depression, anxiety, and embarrassment are linked to worse fatigue. Environmental stressors and unsupportive reactions from others can further impact a person's experience of fatigue.