Fatigue is generally defined as extreme tiredness due to exertion or illness, whereas energy refers to the actual or perceived ability to power physical and mental activity. Fatigue in exercise science is slightly different: fatigue is characterized as diminished output in response to sustained effort.[1] The broad topic of fatigue and energy is related to nutrient status, metabolism and energetics (i.e. mitochondria function), exercise, physiology, psychology, neurobiology, oxidative stress, inflammation, sleep, and many body systems. Fatigue can be as mild as feeling tired regularly, or as extreme as the disabling fatigue indicative of a severe medical condition (i.e. chronic fatigue syndrome (CFS), also known as ME/CFS).[2]
What does energy and fatigue encompass?
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•Last Updated: May 16, 2025
Energy refers to the ability to perform physical and mental activities, and fatigue is defined as extreme tiredness resulting from exertion or illness, as characterized by diminished output during sustained effort. The concepts of energy and fatigue are interconnected with various factors such as nutrient status, metabolism, exercise, and overall health conditions.