People who have not undergone cancer treatment don't understand cancer fatigue. As we all know, it is not the way one feels after a poor night's sleep or working out at the gym or just staying up too late. It is a pervasive feeling of exhaustion and the usual remedies don't help much.
Sometimes it is caused by the treatment itself; it is on the list of possible side effects for most chemo drugs. Sometimes it is caused by low blood counts, and sometimes there are psychological factors such as depression.
This is a helpful summary from ASCO Answers:
What is cancer-related fatigue?
Cancer-related fatigue is a persistent feeling
of physical, emotional, or mental tiredness or
exhaustion related to cancer and/or its treatment.
Unlike other types of fatigue, the feeling does not
go away with rest. Most people receiving cancer
treatment experience fatigue, and some people will
continue to experience fatigue after treatment is
over. If you experience fatigue, it is important to tell
your doctor. Fatigue can cause a person with cancer to
avoid or skip treatments. It may also negatively affect
other areas of life, including mental and physical health,
relationships, and work.
What causes cancer-related
Most people receiving chemotherapy, radiation therapy,
surgery, or other treatments experience fatigue. This
fatigue may be caused by a low level of red blood cells, called anemia, or hormone levels that are too low or too high. Side effects related to
nutrition, such as loss of appetite or dehydration, may result in fatigue, as can a lack of exercise. Anxiety and depression are the most common
psychological reasons fatigue occurs. Pain, stress, lack of sleep, medications, and other medical conditions can also cause fatigue
Read more: http://www.cancer.net/sites/cancer.net/files/asco_answers_fatigue.pdf