Central fatigue: issues related to cognition, mood and behavior, and psychiatric diagnoses.

Describe and define the concept of central fatigue, as well as discuss the behavioral and neuroimaging studies that attempt its assessment and measurement. A brief description of how fatigue can be differentiated from psychiatric symptoms and from sleepiness is also provided. Promising directions for future research will be outlined in the final portion of this review. Despite its popular clinical use as a descriptor of certain symptomatology, fatigue is not a unitary construct. Describing central fatigue as distinct from peripheral fatigue represents one way of understanding the underlying neural and behavioral concomitants of fatigue.