Why do we get headaches when we skip coffee?

Caffeine narrows blood vessels in the brain by blocking adenosine receptors. With daily use, the brain grows extra adenosine receptors to compensate. When you skip your usual dose, adenosine floods these receptors and blood vessels dilate quickly, increasing blood flow and pressure around the brain, which triggers a headache. Withdrawal symptoms can start 12 to 24 hours after the last dose and may include fatigue, irritability and difficulty concentrating. Tapering caffeine gradually over a week or two, rather than stopping abruptly, helps avoid or reduce withdrawal headaches.