Well, according to sharecare, the explanation is that, as the brain creates adenosine it binds to adenosine receptors. Adenosine is one of four nucleoside units in RNA. That binding of adenosine causes drowsiness by slowing nerve cell activity. The adenosine binding also causes the brain's blood vessels to dilate, most likely to let in more oxygen during sleep.
Caffeine looks just like adenosine to a nerve cell. Caffeine therefore binds to the adenosine receptor. But unlike adenosine, it doesn't slow down the cell's activity. As a result, the cell can't identify adenosine -- the caffeine is taking up all the receptors. Instead of slowing down because of the adenosine's effect, nerve cells speed up. The caffeine also causes the brain's blood vessels to constrict. It is, after all, blocking adenosine's ability to open them up. This is why some headache medicines contain caffeine -- if you have a vascular headache, caffeine will close down the blood vessels and offer relief.
Now, you have increased neuron firing in the brain. When the pituitary gland sees all of this activity, it thinks an emergency must be occurring. The pituitary, therefore, releases hormones to tell the adrenal glands to produce adrenaline (epinephrine). Adrenaline, the "fight or flight" hormone, has a number of effects on the body: Pupils dilate. Breathing tubes open (which is why people suffering from severe asthma attacks sometimes can be injected with epinephrine). The heart beats faster. Muscles tighten up, ready for action. Blood pressure rises. Blood flow to the stomach slows. The liver releases sugar into the bloodstream. This explains why, after drinking a big cup of coffee, your muscles tense up, you feel excited, your hands get cold and you can feel your heart beat increasing.
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