Sleep problems; people may still feel weary after full nights of sleep, or may struggle to stay awake, fall asleep or stay asleep.Additionally, one of the following symptoms must be present:
Problems with thinking and memory (cognitive dysfunction, sometimes described as "brain fog")
While standing or sitting upright; lightheadedness, dizziness, weakness, fainting or vision changes may occur (orthostatic intolerance)

Other common symptoms
Many, but not all people with ME/CFS report:
Muscle pain, joint pain without swelling or redness, and headache
Tender lymph nodes in the neck or armpits
Sore throat
Irritable bowel syndrome
Chills and night sweats
Allergies and sensitivities to foods, odors, chemicals, lights, or noise
Shortness of breath
Irregular heartbeatIncreased sensitivity to sensory stimuli and pain have also been observed in CFS.The CDC recommends that people with symptoms of CFS consult a physician to rule out other illnesses, which may be treatable. Onset
The onset of CFS may be gradual or sudden. When it begins suddenly, it often follows a period of infectious-like symptoms or a known infection, and between 20 and 80% of patients report an onset resembling an infection. : 158  When gradual, the illness may begin over the course of months or years. Studies disagree as to which pattern is more common. : 158 : 181  CFS may also occur after physical trauma such as a car accident or surgery. Physical functioning
The functional capacity of individuals with CFS varies greatly. Some persons with mild CFS lead relatively normal lives with vigilant energy management; persons that are severely ill may be totally bed-ridden and unable to care for themselves.
The 2015 definition by the National Academy of Medicine (then referred to as the "Institute of Medicine") is not a definition of exclusion (differential diagnosis is still required). "Diagnosis requires that the patient have the following three symptoms: 1) A substantial reduction or impairment in the ability to engage in pre-illness levels of occupational, educational, social, or personal activities, that persists for more than 6 months and is accompanied by fatigue, which is often profound, is of new or definite onset (not lifelong), is not the result of ongoing excessive exertion, and is not substantially alleviated by rest, and 2) post-exertional malaise* 3) Unrefreshing sleep*; At least one of the two following manifestations is also required: 1) Cognitive impairment* 2) Orthostatic intolerance" and notes that "*Frequency and severity of symptoms should be assessed. The diagnosis of ME/CFS should be questioned if patients do not have these symptoms at least half the time with moderate, substantial, or severe intensity. "Clinical practice guidelines are generally based on case descriptions, with the aim of improving diagnosis, management and treatment. An example is the ME/CFS guideline for the National Health Services in England and Wales, updated in 2021. Other guidance can be found at the New York Department of Health. Differential diagnosis
Certain medical conditions can cause chronic fatigue and must be ruled out before a diagnosis of CFS can be given.