We feel fatigue. Its normal if you really do good amount of physical work. If not, be attentive. Oxidative stress may be behind this fatigue.
Cells are composed of millions of molecule. Sometimes, molecule can have less electron than needed. In this case, such molecule (free radicals) santches electron from nearby molecule. In turn, victim molecule turns to free radicals. It can result in chain reaction causing cell death and it will erode the nearby cells as well.
Normal metabolism
Stress
Poor diet
Lack of exercise
Lack of sleep
Pollution
Consumption of alcohol
Consumption of Nicotine product
It provides electron to free radicals. So it works as neutrilizer.
https://www.youtube.com/watch?v=dlZ5ROca0KI
https://www.youtube.com/watch?v=TOAUuOXjoLs
https://en.wikipedia.org/wiki/Oxidative_stress