How does coffee keep you awake?

By throwing an overnight party.

No.

When we perform our daily activities like thinking and playing, a by product called adenosine is produced.

Adenosine slows down the brain activity.

But how?

In our brain, there are adenosine receptors which are perfectly molded for this adenosine.

When the adenosine binds to these receptors.

It activates them, causing to slow down the brain activity and thus making us feel sleepy.

However, drinking coffee keeps us awake and we don't feel sleepy.

This is because coffee contains a drug called caffeine which after digestion reaches our brain.

Caffeine is structurally similar to adenosine.

Being similar, caffeine binds to the adenosine receptors and thus blocks the adenosine from binding.

Hence, as adenosine does not bind, our receptors don't slow the brain activity.

As a result, we remain awake.