Fatigue is usually caused by too little sleep or overexertion and goes away with sufficient rest.

If it persists, however, it could be a symptom of what is known as Chronic Fatigue Syndrome (CFS).

CFS is a complicated disorder principally characterized by extreme fatigue, according to Fatigatio, a national self-help organization of CFS sufferers in Germany.

Symptoms, aside from the "paralyzing fatigue", include headaches, muscle pain, a sore throat or neck, painful joints and a loss of memory or concentration.

"You feel tired regardless of your biorhythm," explains Peter Falkai, of the German Association for Psychiatry and Psychotherapy. "These people get up in the morning, feel knackered, can't really get in gear and are in a bad mood."

Then comes yet another "dreadful night" during which they sleep poorly, Falkai says.

"They often feel as though they've got a permanent case of the flu," he adds.

What causes CFS is not fully clear. According to Fatigatio, possible triggers include an immune system dysfunction, viruses, hormonal disorders, fungi, psychological factors, stress and environmental influences.

A weak or chronically activated immune system seems to be at the heart of the syndrome.

Wolfgang Huber, a Heidelberg-based internist, nephrologist and environmental physician, says CFS is a systemic inflammatory disorder in which the metabolism and energy supply of cells become imbalanced.

The bodily processes involved in CFS are similar to "a battery cell emptying", Huber says, noting that CFS is often preceded by an infection and a resulting physical breakdown.

Harmful environmental conditions can also play a role, he adds, particularly exposure to mercury in amalgam tooth fillings, as well as wood preservatives and insect repellents.

Diagnosing CFS is difficult, says Wolfgang Wesiack, president of the Professional Association of German Internists.

Because symptoms are unspecific, wide-ranging and often identical to those of other illnesses, organic and psychosomatic causes must be ruled out first, he says.

Trying to pinpoint the source of the patient's suffering "is a little like looking for a needle in a haystack", he says. "Sometimes it takes the doctor months to find the cause."

First of all, Wesiack says, the doctor has to determine whether the patient is exhausted due to heavy physical exertion or a strength-sapping operation. If this is not the case, other possible causes, such as stress or depression, are checked.

"Fatigue is also a primary symptom of depression," Wesiack says.

So, persistent fatigue could be caused by a relative's death, a child's severe illness or constant trouble at the workplace.

Choosing the right therapy for CFS patients is difficult, too. The biggest problem for the treating physician, Wesiack says, is that there is no clear diagnosis for CFS.

And there is no general rule on when, or if, a therapy will take effect, Huber says.

The condition is often very oppressive for sufferers, who must contend not only with its symptoms but also with its social and psychological effects.

Falkai advises people who have had the symptoms for at least 14 days, and whose family doctor can find no cause, to be examined by a psychiatric or psychotherapeutic specialist.

German Press Agency