The fatigue experienced in ME/CFS is pathological and incapacitating.
It is not normal tiredness.

Consideration of “fatigue” as mental or physical tiredness is too simplistic to encompass the scope of impairment in CFS/ME, and belies the inadequacy of the vocabulary of fatigue.

J N Baraniuk, BMJ Best Practice, 2018

The pathological fatigue experienced in ME/CFS can be described as incapacitating weakness, heaviness, or a paralytic exhaustion. While the words, 'tiredness' or 'fatigue', may be used by patients, these do not express the extent and complexity of the symptoms, or the extreme energy depletion, weakness and sickness experienced by patients.

There is sufficient evidence that fatigue in ME/CFS is profound, not the result of ongoing excessive exertion, and not substantially alleviated by rest. This fatigue results in a substantial reduction or impairment in the ability to engage in pre-illness levels of occupational, educational, social, or personal activities ...

IOM Report, 2015, p78

Fatigue in ME/CFS

Researchers have identified the following types of fatigue in ME/CFS:

  • metabolic fatigue limiting the ability of cells to function normally
  • oxygenation fatigue caused by insufficient oxygen being delivered to the brain and tissues
  • arousal fatigue due to poor sleep quality
  • structural fatigue in those with comorbid musculo-skeletal conditions

People with ME/CFS experience these abnormalities in the following ways:

  • debilitating drain of energy
  • abnormal physical exhaustion or muscle weakness following minor physical or cognitive activity
  • mental exhaustion after everyday cognitive effort
  • feeling over-stimulated when exhausted, 'tired but wired'
  • persistent flu-like feelings

The words fatigue and post-exertional malaise (PEM) are sometimes confused. Fatigue is one symptom of the illness, while PEM is the unique, identifying feature of ME/CFS.


Up to 30.5% of the population have chronic fatigue. Therefore, it is necessary to carefully consider diagnostic criteria and exclusionary conditions in the evaluation of a patient with prolonged unremitting fatigue.

J N Baraniuk, BMJ Best Practice, 2018

Assesssment of fatigue in a patient with a possible diagnosis of ME/CFS should include consideration of the differences between chronic tiredness, post-viral fatigue, other fatiguing conditions, and ME/CFS. The key indicator of ME/CFS is post-exertional malaise (PEM).

Standardised fatigue measures rarely have the specificity and sensitivity to identify the different aspects of fatigue in ME/CFS, or to quantify the level or severity of the fatigue, or its fluctuating nature.

It is important to identify any causes of fatigue that are additional to the ME/CFS and which may respond to treatment. These are many and varied, and include a variety of anaemia and thyroid conditions.

Situational depression is common in ME/CFS due to the losses and life changes that accompany the illness. ME/CFS symptoms may be mistakenly interpreted by standard questionnaires as indicators of anxiety and depression, resulting in incorrect diagnosis and treatment. Treatment plans must reflect the reported experiences of the individual, rather than relying on test scores.

Because of the complex nature of fatigue in ME/CFS, a detailed history is very important. This may be the best tool for identifying the nature and impact of fatigue in a person's life.

Management and treatment

The complexity of fatigue in ME/CFS requires close attention to causal factors.

Even mild ME/CFS can cause a 50% loss of normal daily functional capacity. It is important that individuals with ME/CFS learn to recognise the limit of their capacity for the day and not push beyond this. See our page on pacing, for information on how to safely manage rest and activity.

Fatigue is what we experience, but it is what a match is to an atomic bomb.

Laura Hillenbrand, author of Seabiscuit and Unbroken, New York Times, 4 Feb 2011

Last edited: 10 December, 2020