At some point, nearly everyone suffers from fatigue. In most cases, it's acute fatigue -- fatigue that occurs suddenly but lasts less than three months and is usually caused by lifestyle or environmental factors such as physical exertion, stress, lack of sleep, dehydration, or inadequate diet. In most cases, it's easily treated by addressing the cause: by reducing stress; getting more rest, sleep, or better nutrition; or hydrating properly.

Chronic fatigue lasts longer -- for more than a few months -- and it's more likely to be associated with an underlying medical condition.

Studies have consistently shown that women are more likely than men to experience fatigue. Women also are four times as likely as men to develop chronic fatigue syndrome.

Fatigue refers to a lack of energy. It is different from weakness, which refers to a loss of strength or power, and drowsiness, because you can feel tired without feeling drowsy or sleepy. Although people sometimes use these terms interchangeably, from a medical perspective the symptoms are distinctly different.

Chronic fatigue is one aspect of the specific medical condition chronic fatigue syndrome, but CFS involves more than just fatigue. You can have chronic fatigue without having chronic fatigue syndrome. CFS is extreme, long-term fatigue that is associated with other multiple symptoms. Also, in CFS, other possible causes of fatigue have been ruled out.

Fatigue is sometimes a side effect of psychoactive substances such as alcohol, nicotine, and caffeine. Other substances that can cause fatigue include many over-the-counter and prescription medications, including many antihistamines, blood pressure medications, steroids, tranquilizers, and painkillers as well as marijuana and many other illegal drugs.

Caffeine is known for its stimulating effects, but long-term heavy use can result in fatigue, especially during periods of withdrawal.

Fatigue is a common feature of many, if not most, illnesses. This includes all of the above as well as cancer; anemia; multiple sclerosis; eating disorders; autoimmune diseases (such as lupus); heart, liver, or kidney disease; and a wide variety of infections.

Fatigue is associated with a wide variety of mental, emotional, or psychological issues. It is both a symptom of depression and a risk factor for it. It's also associated with grief, stress, anxiety, and substance abuse.

Hypothyroidism tends to make people feel tired, sluggish, and depressed.

It is a condition in which the thyroid gland does not make enough thyroid hormone. Inflammation of the thyroid gland is the most common cause. Other symptoms are being more sensitive to cold, depression, heavier menstrual periods, weakness, and unintentional weight gain.

Exercise can often help relieve the effects of fatigue, especially more short-lived, acute forms of fatigue. Other things that may help: better hydration, better diet, relaxation, and, of course, more rest and better sleep.

Caffeine and other stimulants may help fight fatigue in the short term, but longer-term, they may actually contribute to the problem.

In most cases, chronic fatigue is not substantially improved by rest. Chronic fatigue often makes it more difficult to get adequate rest. Similarly, fatigue can interfere with sleep, resulting in unrefreshing sleep, difficulty falling asleep, frequent awakening, or sleep apnea.

Rest is unlikely to relieve fatigue caused by a medical condition.

Fatigue is one of the most common complaints in medicine, accounting for an estimated 10 million doctor office visits each year.

Because fatigue is a common feature of everyday life but also a common symptom of so many different medical conditions, it can be difficult for doctors to properly assess and treat. In many cases, the most important medical response to persistent fatigue is to get tested for potential underlying physical or psychological medical conditions.

Chronic fatigue syndrome is a term used to describe a group of symptoms, with fatigue being the most prominent. Other key symptoms include muscle and joint pain, sleep disturbances, headaches, sore throat, and tender lymph nodes. According to the CDC, these symptoms can be as disabling as multiple sclerosis, lupus, rheumatoid arthritis, heart disease, end-stage renal disease, chronic obstructive pulmonary disease (COPD), and similar chronic conditions.

Fatigue also is frequently a symptom of conditions that are distinct from chronic fatigue syndrome but are often associated with it -- including some autoimmune disorders and fibromyalgia.

The cause of chronic fatigue syndrome is unknown. Researchers are investigating a number of possibilities, including infections, stress, immune system dysfunction, and nutritional deficiencies.

There is no blood test for chronic fatigue syndrome. However, if you have experienced unexplained fatigue lasting longer than three months, you may want to see your doctor to rule out possible medical problems. Chronic fatigue syndrome may be considered if you have experienced fatigue for longer than six months -- especially if you have chronic pain or other common symptoms of CFS. Diagnosis is usually based on an evaluation of the patient's history and a thorough physical and mental examination that rules out other potential causes.

While exercise can be helpful for more common forms of fatigue, the relationship of exercise to chronic fatigue syndrome is much more complex.  Exercise can, in fact, worsen the symptoms of CFS -- a phenomenon sometimes referred to as "post-exertional malaise" or "payback." This can strike CFS patients about 12 to 48 hours after exercise and make their symptoms worse for days or even weeks.

Even so, CFS patients are not discouraged from exercising. A carefully structured and balanced program of exercise can, over time, help build stamina.

A daily 30-minute walk or other moderate aerobic exercise has shown more consistently positive benefit for fatigue symptoms than any other treatment studied. However, exercise can temporarily worsen symptoms of chronic fatigue syndrome, so CFS patients should see their doctor before beginning an exercise program.