I normally drink about ten cups of coffee per day but today I decided to cut back and haven't had any caffeine but now my head hurts and I don't know why do YOU know why!?

Caffeine! : https://www.youtube.com/watch?v=Xl1XBJLfIDU

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Sources:
https://www.sciencedaily.com/releases/2009/05/090501162805.htm
https://www.ncbi.nlm.nih.gov/pubmed/26677204
https://www.ncbi.nlm.nih.gov/pubmed/15448977
https://www.ncbi.nlm.nih.gov/pubmed/18625110
http://www.smithsonianmag.com/science-nature/this-is-how-your-brain-becomes-addicted-to-caffeine-26861037/
http://bonhamchemistry.com/wp-content/uploads/2012/01/Caffeine_and_Adenosine.pdf
http://www.sciencedirect.com/science/article/pii/S0074774205630073
https://www.ncbi.nlm.nih.gov/books/NBK223808/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3437321/
https://www.ncbi.nlm.nih.gov/pubmed/1846425
http://www.apa.org/monitor/2008/04/caffeine.aspx

Images:
https://commons.wikimedia.org/wiki/File:Caffeine_(1)_3D_ball.png
https://commons.wikimedia.org/wiki/File:Caffeine_and_adenosine.svg
https://commons.wikimedia.org/wiki/File:Caffeine_and_adenosine.svg