Fatigue is generally defined as a feeling of lack of energy and motivation that can be physical, mental or both. Fatigue is not the same as drowsiness, but the desire to sleep may accompany fatigue. Apathy is a feeling of indifference that may accompany fatigue or exist independently. In addition, individuals often describe fatigue using a variety of terms including weary, tired, exhausted, malaise, listless, lack of energy and feeling run down.
Fatigue is common. About 20% of Americans claim to have fatigue intense enough to interfere with living a normal life. A physical cause has been estimated to be responsible 20% to 60% of the time, while emotional or mental causes comprise the other 40% to 80% of cases of fatigue. Unfortunately, fatigue can also occur in normal individuals that experience intense physical or mental activity (or both).
However, in contrast to fatigue that occurs with some diseases and syndromes, normal fatigue in healthy individuals is quickly relieved in a few hours to about a day when the physical or mental activity is reduced. Also, people occasionally experience fatigue after eating (sometimes termed postprandial depression) which can be a normal response to food, especially after large meals and this may last about 30 minutes to several hours.
In addition to the many terms attributed to "fatigue", there are further problems with the terminology used to describe fatigue. There are several "fatigue syndromes" that occasionally appear in the medical literature. For example, Epstein-Barr chronic fatigue syndrome, post viral infection fatigue syndrome, and adrenal fatigue syndrome are among the most commonly seen. However, many physicians do not recognize these as syndromes because the criteria used to define them as syndromes are too diffuse and many consider the associated fatigue (sometimes chronic fatigue) as either a symptom or complication of the underlying associated diseases. However, there is a well-defined chronic fatigue syndrome recognized by specific criteria. Basically, two sets of criteria need to be met to establish a diagnosis of chronic fatigue syndrome:
1. Have severe chronic fatigue for at least six months or longer with other known medical conditions (whose manifestation includes fatigue) excluded by clinical diagnosis; and
2. Concurrently have four or more of the following symptoms:
* post-exertional malaise
* impaired memory or concentration
* unrefreshing sleep
* muscle pain
* multi-joint pain without redness or swelling
* tender cervical or axillary lymph nodes
* sore throat
Consequently, people and their health care practitioners need to spend some time together to clearly determine whether or not the problem or symptom is truly fatigue, and if it is, any associated symptoms that may accompany the fatigue should be explored.
Source : http://www.emedicinehealth.com/fatigue/article_em.htm