What is Chronic Fatigue Syndrome?

Medically Reviewed by Jennifer Robinson, MD on November 15, 2022

Everyone feels fatigued sometimes, and many people feel fatigued a lot of the time.

But myalgic encephalomyelitis / chronic fatigue syndrome (ME/CFS) isn't just being tired. It's a new state of fatigue that has lasted for at least 6 months, and can be so severe that it gets in the way of your normal daily activities, at home and at work. Rest and sleep don't seem to help.

If you have it, physical activity can leave you feeling worse, typically the next day. This condition is known as post-exertional malaise.

You tend to awaken almost every morning feeling as if you didn't get enough sleep Often you wake up a lot at night, for no apparent reason.

You can have trouble concentrating and multi-tasking.

When you stand upright from a sitting or lying position, you may feel light-headed. Your heart might beat rapidly.  After you're on your feet for a while, you could feel terrible, and need to lie down flat.

ME/CFS tends to cycle through flareups and remissions. You'll have good days and bad days, although on the good days you won't feel back to normal. There isn't any known cure, but different treatments can help the symptoms.

Facts about Chronic Fatigue Syndrome

  • The CDC estimates up to 2.5 million Americans have ME/CFS.
  • Anyone can get it, including children and teens.
  • It's most common in women in their 40s and 50s.
  • Women are more likely to develop it than men.
  • Most cases are mild or moderate.
  • About 1 in 4 people with the condition have severe symptoms.

If you have mild ME/CFS, you can probably manage on your own. Moderate symptoms can make it hard for you to move around. For example, you might need to sleep in the afternoons.

Severe symptoms can impact your quality of life and abilities as badly as if you had lupus, heart disease, or rheumatoid arthritis.

How Does It Happen?

Doctors don't know what causes ME/CFS, but they have identified different underlying abnormalities in people with the condition.

Immune system problems: Several parts of the immune system are different in people with ME/CFS. Some research indicates that these abnormalities may cause the symptoms of the illness. Fortunately, people with ME/CFS don't have a defective immune system in the way that people with HIV/AIDS do.

Energy production: In you have ME/CFS, the cells in your body have trouble making enough energy.

Brain abnormalities: Abnormalities show up in pictures of the brain (like MRI or CT scans), in levels of brain hormones, and in the brain's electrical system (brain waves). These abnormalities can come and go, and aren't necessarily permanent.

Blood pressure and pulse issues: When you stand, you blood pressure could drop and your heart might start to beat faster. You might feel faint or pass out if your blood pressure drops too low.

Genes: Some studies have found abnormalities in the structure of certain genes. Others point to abnormalities in the way certain genes are turned on and off inside your cells. Studies of identical and non-identical twins indicate that some people inherit a genetic likelihood for getting the illness.

Infections or other illness: ME/CFS often, but not always, begins with a sudden infectious-like illness (fever, sore throat, aching muscles, upset stomach). Research found that several different types of infectious agents can trigger the beginning of the illness, including Epstein-Barr virus (a common cause of mononucleosis), Lyme disease bacteria and Q fever bacteria.

Serotonin and cortisol: Several studies indicate that serotonin, a major brain chemical, plays an important role in ME/CFS symptoms. People with the illness have low levels of cortisol, a hormone the body releases in response to stress.

How Can I Find Out If I Have It?

The National Academy of Medicine has issued guidelines describing the combination of symptoms necessary for doctors to diagnose the condition. Unfortunately, there isn't yet a diagnostic test that's accurate enough to be useful.

Because extreme fatigue is a symptom of so many conditions, your doctor will want to rule out other conditions first, before considering a diagnosis of ME/CFS. You'll need to get a complete checkup and talk with your doctor about all your symptoms.

Show Sources


CDC: “Chronic Fatigue Syndrome.”

WebMD Medical Reference: "Coxsackie Virus."

Office of Women's Health, U.S. Department of Health and Human Services: “Chronic fatigue syndrome.”

National Health Service (U.K.): “Chronic fatigue syndrome.”

Institute of Medicine. Beyond Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Redefining an Illness. (2015)

Tomas, C., et al.. "Cellular bioenergetics is impaired in patients with chronic fatigue syndrome." PloS One (2017)

Armstrong, C. W., et al. (2015). "Metabolic profiling reveals anomalous energy metabolism and oxidative stress pathways in chronic fatigue syndrome patients." Metabolomics (2015).

Komaroff, A. L. and T. A. Cho. "Role of infection and neurologic dysfunction in chronic fatigue syndrome." Seminars in Neurology (2011)

Montoya, J. G., et al.. "Cytokine signature associated with disease severity in chronic fatigue syndrome patients." Proceedings of the National Academy of Sciences of the United States of America (2017)

Wang, T., et al.. "A systematic review of the association between fatigue and genetic polymorphisms." Brain, Behavior, and Immunity (2017) 

Hornig, M., et al.. "Distinct plasma immune signatures in ME/CFS are present early in the course of illness." (2015)

Hornig, M., et al. "Immune network analysis of cerebrospinal fluid in myalgic encephalomyelitis/chronic fatigue syndrome with atypical and classical presentations." Transl Psychiatry (2017)

Whistler, T., et al.. "Gene expression correlates of unexplained fatigue." Pharmacogenomics (2006)

Light, A. R., et al. (2012). "Gene expression alterations at baseline and following moderate exercise in patients with Chronic Fatigue Syndrome and Fibromyalgia Syndrome." Journal of Internal Medicine (2012)

Razumovsky, A. Y., et al.. "Cerebral and systemic hemodynamics changes during upright tilt in chronic fatigue syndrome." Journal of Neuroimaging (2003)

Hickie, I., et al.. "Post-infective and chronic fatigue syndromes precipitated by viral and non-viral pathogens: prospective cohort study." (2002)

Schutzer, S. E., et al.. "Distinct cerebrospinal fluid proteomes differentiate post-treatment Lyme disease from chronic fatigue syndrome." (2011)

Katz, B. Z., et al.. "Chronic fatigue syndrome after infectious mononucleosis in adolescents." Pediatrics (2009)

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