Why a headache after a day of not drinking coffee?

Caffeine helps your body "kick start" so to speak. It gives you energy, but once you become more reliable on that extra boost, your body craves it. So, when you get rid of something that makes you less tired, you're probably going to get a headache. Believe it or not, caffeine will help get rid of your head ache. So it's okay to have a cup of coffee in the morning, but just don't drink too much or it's bad for you. Hope this helps:)

Caffeine is a drug, when you have to long without it, your body carves it.