For the entire time that caffeine is in your system, the sleepiness chemical it blocks (adenosine) nevertheless continues to build up. Your brain is not aware of this rising tide of sleep-encouraging adenosine, however, because the wall of caffeine you’ve created is holding it back from your perception. But once your liver dismantles that barricade of caffeine, you feel a vicious backlash: you are hit with the sleepiness you had experienced two or three hours ago before you drank that cup of coffee plus all the extra adenosine that has accumulated in the hours in between, impatiently waiting
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