Here is what happens when you drink coffee:

• Caffeine enters your bloodstream within minutes and starts blocking adenosine receptors. The effect often peaks within about 45 minutes.
• It also stimulates your sympathetic nervous system, prompting the adrenal glands to release adrenaline — which is why caffeine can make you jittery
• Caffeine's half-life averages about 5-6 hours, though genetics shift this widely from person to person. A coffee at 4pm can leave half the dose still active at 10pm
• With daily use, tolerance can develop; adenosine-system changes are one proposed contributor, but the adaptation is not a single settled mechanism. Skip caffeine abruptly, and adenosine signaling can contribute to withdrawal headaches and fatigue