Chronic Fatigue Syndrome (ME/CFS): What It Is, What It Feels Like, and When to Get Help

What are the core features of ME/CFS?

The 2015 National Academy of Medicine (formerly IOM) diagnostic criteria require all three of the following to be present for at least six months, occurring at least half the time at a moderate, substantial, or severe level 1Ref 1Committee on the Diagnostic Criteria for Myalgic Encephalomyelitis/Chronic Fatigue Syndrome, Board on the Health of Select Populations, Institute of Medicine (2015).Beyond Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Redefining an Illness.ME/CFS diagnostic criteria requiring profound fatigue, post-exertional malaise, and unrefreshing sleep for at least six months; the spectrum of severity; graded exercise therapy concerns:

1. Profound fatigue that is new (not lifelong), not the result of effort, and not substantially relieved by rest — a fatigue that meaningfully reduces the ability to do activities that were possible before illness.

2. Post-exertional malaise (PEM): A worsening of symptoms following physical or cognitive activity, often delayed by 12–48 hours. The 'crash' can include increased fatigue, brain fog, pain, and flu-like feelings lasting days. PEM is considered the most distinctive feature of ME/CFS and sets it apart from most other causes of chronic fatigue 1Ref 1Committee on the Diagnostic Criteria for Myalgic Encephalomyelitis/Chronic Fatigue Syndrome, Board on the Health of Select Populations, Institute of Medicine (2015).Beyond Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Redefining an Illness.ME/CFS diagnostic criteria requiring profound fatigue, post-exertional malaise, and unrefreshing sleep for at least six months; the spectrum of severity; graded exercise therapy concerns.

3. Unrefreshing sleep: Waking from sleep feeling as tired as or more tired than before going to bed, regardless of hours slept.

Who develops ME/CFS and what triggers it?

ME/CFS affects people of all ages, sexes, and backgrounds, though it is more common in adults aged 40–60 and significantly more common in women than men. An estimated 3.3 million people in the United States have ME/CFS, but more than 9 in 10 remain undiagnosed — often because they have not yet seen a clinician familiar with current criteria 4Ref 4Centers for Disease Control and Prevention (2024).ME/CFS Basics.Up to 3.3 million Americans have ME/CFS; >90% undiagnosed; pacing/activity management as the recommended self-management approach; vigorous exercise not tolerated and not recommended.

A substantial proportion of cases begin after an acute infection — viral or bacterial. This post-infectious pathway gained visibility with Long COVID, in which a significant minority of people who recover from COVID-19 develop persistent fatigue, post-exertional worsening, and cognitive difficulties that resemble ME/CFS closely. Some clinicians now evaluate and manage post-COVID conditions alongside or within the ME/CFS framework.

What other symptoms are common in ME/CFS?

Most people with ME/CFS also experience at least one of the following:

• Cognitive impairment (often called 'brain fog') — difficulty thinking, concentrating, finding words, or remembering
• Orthostatic intolerance — symptoms that worsen when upright, including dizziness, rapid heartbeat, weakness, or fainting

Other frequent but not universal symptoms include widespread pain or headaches, sensitivity to light or sound, temperature dysregulation, sore throat, and tender lymph nodes.

ME/CFS exists on a spectrum — some people are mildly affected and can still work; others are severely disabled and mostly bedbound 1Ref 1Committee on the Diagnostic Criteria for Myalgic Encephalomyelitis/Chronic Fatigue Syndrome, Board on the Health of Select Populations, Institute of Medicine (2015).Beyond Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Redefining an Illness.ME/CFS diagnostic criteria requiring profound fatigue, post-exertional malaise, and unrefreshing sleep for at least six months; the spectrum of severity; graded exercise therapy concerns.

How is ME/CFS diagnosed — and why does it take time?

There is no specific blood test or scan for ME/CFS. Diagnosis involves a careful history, physical exam, and lab work to exclude other treatable conditions before ME/CFS is confirmed 1Ref 1Committee on the Diagnostic Criteria for Myalgic Encephalomyelitis/Chronic Fatigue Syndrome, Board on the Health of Select Populations, Institute of Medicine (2015).Beyond Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Redefining an Illness.ME/CFS diagnostic criteria requiring profound fatigue, post-exertional malaise, and unrefreshing sleep for at least six months; the spectrum of severity; graded exercise therapy concerns. Conditions that must be ruled out include:

• Hypothyroidism — closely mimics ME/CFS and responds well to treatment; detected with a TSH blood test 2Ref 2Jonklaas J, Bianco AC, Bauer AJ, et al. (2014).Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement.Hypothyroidism as a key treatable condition to rule out before confirming ME/CFS, given overlapping fatigue presentation
• Obstructive sleep apnea — a sleep study can rule this in or out 3Ref 3Kapur VK, Auckley DH, Chowdhuri S, et al. (2017).Clinical Practice Guideline for Diagnostic Testing for Adult Obstructive Sleep Apnea: An American Academy of Sleep Medicine Clinical Practice Guideline.Obstructive sleep apnea as an important differential diagnosis in ME/CFS evaluation, detectable with sleep study
• Anemia and nutritional deficiencies
• Autoimmune conditions (lupus, rheumatoid arthritis)
• Depression — can co-occur with ME/CFS but is a separate condition

This process can be lengthy and frustrating. ME/CFS is not the same as depression, not caused by deconditioning, and not a diagnosis of exclusion that means 'nothing is wrong.' It is a physiological condition with growing evidence of immune and autonomic nervous system involvement 4Ref 4Centers for Disease Control and Prevention (2024).ME/CFS Basics.Up to 3.3 million Americans have ME/CFS; >90% undiagnosed; pacing/activity management as the recommended self-management approach; vigorous exercise not tolerated and not recommended.

Up to 3.3 million people in the United States have ME/CFS, and more than 90% remain undiagnosed 4Ref 4Centers for Disease Control and Prevention (2024).ME/CFS Basics.Up to 3.3 million Americans have ME/CFS; >90% undiagnosed; pacing/activity management as the recommended self-management approach; vigorous exercise not tolerated and not recommended. Long COVID shares substantial overlap with ME/CFS in symptoms and is sometimes diagnosed alongside it, particularly when onset followed a COVID-19 infection with persistent fatigue and post-exertional worsening weeks to months later.

Why is exercise advice for ME/CFS different from most conditions?

For most chronic conditions, graded exercise is beneficial. ME/CFS is a recognized exception. Graded exercise therapy — once recommended — has been shown to worsen symptoms in people with ME/CFS specifically because of post-exertional malaise. Current guidance has shifted away from this approach 1Ref 1Committee on the Diagnostic Criteria for Myalgic Encephalomyelitis/Chronic Fatigue Syndrome, Board on the Health of Select Populations, Institute of Medicine (2015).Beyond Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Redefining an Illness.ME/CFS diagnostic criteria requiring profound fatigue, post-exertional malaise, and unrefreshing sleep for at least six months; the spectrum of severity; graded exercise therapy concerns4Ref 4Centers for Disease Control and Prevention (2024).ME/CFS Basics.Up to 3.3 million Americans have ME/CFS; >90% undiagnosed; pacing/activity management as the recommended self-management approach; vigorous exercise not tolerated and not recommended.

Instead, pacing — carefully managing activity to stay within one's current energy envelope and avoid triggering crashes — is the cornerstone of current self-management 4Ref 4Centers for Disease Control and Prevention (2024).ME/CFS Basics.Up to 3.3 million Americans have ME/CFS; >90% undiagnosed; pacing/activity management as the recommended self-management approach; vigorous exercise not tolerated and not recommended. This is an important reason to pursue an accurate diagnosis rather than assuming fatigue reflects deconditioning and pushing through it. Pacing strategies must be individually tailored: what is tolerable for one person may trigger a crash in another.