Endorphins are very closely linked to addictions and a mixture of two words:

*         'endogenous' meaning from within the body; and

*         'morphine' a powerful pain fighting drug that is also used and abused for recreational purposes.

The human brain contains its own neuro-chemicals; more potent than heroin, opium and morphine.  Endorphins released under stressful exercise create a 'high'.  Endorphins are also motivational assisting you feel energized for the rest of the day tingly and as if I am floating - refuelling the body both physically and mentally. 

Explained in Section 15 and definitions Overdose Human Induced Drug Only  and  Adrenaline.