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.
Re-entry ventricular arrhythmia is a type of paroxysmal tachycardia occurring in the  ventricle where the cause of the arrhythmia is due to the electric signal not completing the normal circuit, but rather an alternative circuit looping back upon itself. There develops a self-perpetuating rapid and abnormal activation. ("Circus Movement" is another term for this.) Conditions necessary for re-entry include a combination of unidirectional block and slowed conduction. Circus movement may also occur on a smaller scale within the AV node (dual AV nodal physiology), a large bypass tract is not necessary.Re-entry is divided into two major types: [Anatomically Defined] re-entry and [Functionally Defined] re-entry. The circus movement can occur around an anatomical or functional core. Either type may occur alone, or together.Anatomically defined re-entry has a fixed anatomic pathway. Anomalous conduction via accessory pathways (APs) creates the re-entry circuit (which are also called bypass tracts), that exists between the atria and ventricles. Wolff–Parkinson–White syndrome (WPW) is an example of anatomically defined re-entry. WPW syndrome is an atrioventricular re-entrant tachycardia (AVRT), secondary to an accessory pathway that connects the epicardial surfaces of the atrium and ventricle along the AV groove. The majority of time symptomatic WPW fits the definition of AVRT (Supraventricular tachycardia) however AVNRT (dual AV nodal physiology) exist in ~10% of patients with WPW syndrome creating the possibility of spontaneous atrial fibrillation degenerating into ventricular fibrillation (VF).