What is lethargy? Lethargy can be described as tiredness, weariness, fatigue, or lack of energy. It can be accompanied by depression, decreased motivation, or apathy. Lethargy can be a normal response to inadequate sleep, overexertion, overworking, stress, lack of exercise, or boredom. When part of a normal response, lethargy often resolves with rest, adequate sleep, decreased stress, and good nutrition. Persistent lethargy that does not resolve with self-care may be an indication of an underlying physical or psychological disorder. Common causes include allergies, asthma, anemia, cancer and its treatments, chronic pain, heart disease, infection, depression, eating disorders, grief, sleeping disorders, thyroid problems, medication side effects, alcohol use, or drug use. The cause of lethargy may be suggested by its pattern and accompanying symptoms. If it starts in the morning and lasts all day, it could be due to lack of sleep or depression. If it develops as the day passes and is accompanied by dry skin, constipation, cold sensitivity, and weight gain, it may be caused by an underactive thyroid gland. The combination of shortness of breath and lethargy could be due to heart or lung problems. Persistent lethargy with no clear diagnosis may result from chronic fatigue syndrome, which can start with a flu-like illness and is often not relieved with rest. The first step in establishing the cause of lethargy is an examination by a physician. The goal of any clinical evaluation for lack of energy is to identify the root cause(s) for the condition. Diagnostic testing may be necessary in order to establish a definitive diagnosis. These tests may include blood and urine tests, imaging tests, and, in some cases, referral to a specialist. The treatment and prognosis of lethargy depend on the underlying cause. Lethargy by itself is rarely an emergency; however, if it develops suddenly or is accompanied by other serious symptoms it may require immediate evaluation to avoid significant complications. Seek immediate medical care (call 911) for sudden energy loss, dizziness, chest pain, confusion, blurred vision, high fever, decreased urine output, sudden swelling or weight gain, shortness of breath, rapid heart rate, altered level of consciousness, severe pain, or if you think you might be a danger to yourself or others. If your lethargy is persistent or causes you concern, seek prompt medical care.