That’s where caffeine comes in. It is shaped very similarly to the adenosine molecule, so it fits into the receptors. The adenosine can’t park where it’s supposed to, because caffeine is already sitting in its parking spots. (That “first sip feeling” Starbucks advertises on the side of its cups sounds a lot more appealing than “blocking morning adenosine,” but that’s what’s really happening.) In truth, caffeine doesn’t pep you up—it simply prevents you from feeling lethargic. Consume enough caffeine, and you’ll have almost no adenosine plugging into your receptors at all, so you’ll feel wired and jittery.
