Caffiene is considered to be a psycho stimulant.

It blocks adenosine — a chemical that helps people fall asleep — from adhering to its receptors in the brain.

Adenosine helps open up blood vessels. Blocking it constricts the brain’s blood vessels, which injects adrenaline into the system. This results in a ‘coffee boost’.

It increases the release of dopamine in the centre of the brain.

Dopamine is a neurotransmitter that activates pleasure centres in certain parts of the brain.

It increases alertness and provides a mental boost.

It improves performance for a short time on a variety of learning, memory, thinking and attention tasks.

Caffeine can increase anxiety and lead to the development of anxiety disorders.

Sources:
     Coffee, Tea, Chocolate and the   
     Brain edited by Astrid Nehlig