There are few things that make you feel as weak and feeble as getting a headache because you missed your morning cup or ten of coffee. No one wants hard evidence they absolutely need coffee to function. However, this isn't uncommon. Around 80-85 percent of Americans consume caffeine regularly.
The headaches happen because you're addicted caffeine. As SciShow notes in the video above, your brain will experience changes in blood flow and electricity when you stop feeding it caffeine. Those changes may result in the splitting headaches you're familiar with. It can also produce a good bout of drowsiness. If you're trying to kick caffeine for good, the withdrawal may last just a couple days. Though, it can also last a couple weeks. Yay!
Caffeine blocks the chemical adenosine from telling receptors in your brain it's time to "hit the brakes." It plays an important role in sleep and energy transfer in your body. Studies suggest if you regularly consume caffeine, your brain eventually creates more adenosine receptors. It's a response to caffeine constantly interfering with normal function. That means if you miss your morning coffee, you can feel increased drowsiness because there are more receptors for adenosine.