https://www.chemistryhelpcenter.org/caffeine/ if you're curious. Caffeine is a really interesting molecule.

The effect of caffeine is related to its structural similarity to adenosine. Adenosine is a nucleotide and is important for coding genes, but it is also used for energy in the form of ATP and as a neuromodulator and signaling molecule. When the brain is active, it consumes lots of ATP (adenosine triphosphate) as an energy source. As the ATP is used it leaves behind a byproduct, AMP (adenosine monophosphate). The longer the brain remains active, the more AMP builds up over time. Specialized adenosine receptors detect the increasing levels of AMP and send a signal that reduces alertness and increases drowsiness until eventually, you fall asleep. While sleeping, the brain has a chance to clear out the AMP and you wake up feeling alert and rested.

Because of caffeine’s structure, it binds to adenosine receptors similarly to adenosine in AMP. However, when caffeine is bound to the receptor, it doesn’t send the sleepy signal. Instead, it blocks AMP from being able to get to that spot. In this way, caffeine essentially “turns off” adenosine receptors and they cannot tell the brain it needs to rest. The more caffeine you take, the more receptors are “turned off” giving the brain the illusion that the AMP has been flushed out resulting in an alert and rested feeling as if you had just woken up.