A wide range of physical and psychological conditions can cause fatigue. Among them are:

• Allergies, asthma and respiratory disorders.
  
  
• Anemia (insufficient number of red blood cells).
  
  
• Cancer. Many cancer treatments also cause fatigue.
  
  
• Cardiovascular disorders such as heart failure, coronary artery disease and endocarditis.
  
  
• Digestive problems including inflammatory bowel disease and liver disease.
  
  
• Emotional disorders including depression, eating disorders and drug or alcohol abuse.
  
  
• Endocrine conditions such as diabetes, thyroid disorders and Addison's disease (underactive adrenal glands).
  
  
• Kidney diseases including chronic renal failure, and treatments including dialysis.
  
  
• Many infectious conditions. Examples include colds, flu, encephalitis, HIV/AIDS, Lyme disease, mononucleosis, parasitic infection, rabies, tuberculosis and West Nile virus.
  
  
• Neurological diseases such as multiple sclerosis.
  
  
• Many pain conditions. These include arthritis, chronic fatigue syndrome, fibromyalgia, headaches, lupus, myofascial pain syndrome, myositis, polymyalgia rheumatica, sarcoidosis, sickle cell anemia and temporal arteritis.
  
  
• Sleep disorders including insomnia, narcolepsy, night terrors and sleepwalking, and sleep-disrupting conditions ranging from apnea to restless legs syndrome to prostate enlargement (benign prostatic hyperplasia).
  

Other sources of fatigue include poor eating habits, malnutrition, excess consumption of caffeine or alcohol, dehydration and pregnancy.

Medication is another factor that may contribute to fatigue. A number of prescription and over-the-counter medications can cause fatigue, including antidepressants, antihistamines, antihypertensives (drugs used to treat high blood pressure), anticonvulsants, chemotherapy drugs, corticosteroids, sleeping pills and triptans (a class of migraine drugs).