Molecular compatibility – one of the reasons we like coffee so much is that there is sympathy between human and coffee biochemistry at a molecular level.
Many of the energising effects of the caffeine contained in coffee are due to its interaction with adenosine receptors in the brain, which play important role in energy transfer.
When you are awake, the neurons in your brain are continually firing, and a by-product of this firing is adenosine, a biochemical compound that is a neuromodulator for the central nervous system. Your neurvous system receptors are constantly monitoring your levels of adenosine, and when they get too high, your brain will slow down neural activity and dilate the blood vessels, making you feel sleepy or crave rest.
Caffeine has a similar molecular structure to adenosine – notably, two nitrogen rings. This similarity in structure means that caffeine can bind to your nervous system’s adenosine receptors without activating then, effectively blocking the receptors from detecting the levels of adenosine and, therefore, keeping you alert even if those levels may be elevated.