Dan drinks a cup or two of strong coffee in the morning to him wake up and get rid of his sleepiness. according to the in focus box: what you really want to know about sleep, the common stimulant drug in coffee, caffeine, blocks receptors for a naturally occurring compound in the body called thus promoting wakefulness.

Adenosine.

Explanation:

Adenosine is a neurotransmitter that is created in the brain and when it binds to the adenosine receptors, it slows down nerve activity and it creates a sensation of drowsiness.

Caffeine looks like adenosine to the receptors, therefore, when we have caffeine, it binds to these receptors and there is no room left for the adenosine to bind. However, the caffeine doesn't slow down nerve activity but, in the contrary, it speeds it up.

Therefore we can tell that caffeine blocks the receptors for the adenosine thus promoting wakefulness.