Required symptoms Substantial reduction or impairment in the ability to engage in pre-illness levels of activity (occupational, educational, social, or personal life) with profound fatigue of new onset, which is present for at least 6 months, is not explained by ongoing or unusual excessive exertion and is not substantially relieved by rest Post-exertional malaise (PEM) Unrefreshing sleep At least one of the following: Cognitive impairment Orthostatic intolerance