What does energy and fatigue encompass?

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]