In short, it's all about two words: Brain Chemistry.
You see, our “Brain Chemistry” have a bunch of neural chemicals in our brains that are in charge of regulating this balance and keeping us awake or asleep and controlling our level of energy, alertness, fatigue, etc.
Caffeine is a fat-soluble chemical that easily passes from your bloodstream into your brain tissue. Our brain activity is controlled by chemicals called neurotransmitters, which either stimulate brain cells or quiet them.
Adenosine is an important one of those “quieting neurotransmitter”. Adenosine is an “inhibitory neurotransmitter” that pushes you towards the “fatigued/sleepy” end of the spectrum. In short: more adenosine means more sleepy. If you have lots of adenosine in there, you are going to feel sleepy, fatigued etc. If you have less adenosine it means you will feel less sleepy.