Coffee is mildest stimulant which increases (to some level) the activity of our central nervous system (CNS). It do it so, by stimulating release of non-adrenaline. It can be used as a cardiac as well as nervous stimulant. But, excessive use can cause addiction.
How to know you are addicted?
It’s addiction is known as caffeinism & intoxication characterized by restlessness, nervousness, excitement, insomnia, gastro-intestinal disorders, headache.