Just a nitpick: caffeine doesn't clear your system of adenosine, it plugs its receptors without activating them, effectively crowding it out. Your brain, with

Just a nitpick: caffeine doesn't clear your system of adenosine, it plugs its receptors without activating them, effectively crowding it out. Your brain, with its countless centuries of evolution, quickly catches on to these shenanigans. After a few days of caffeine use, it will adapt by growing more adenosine receptors -- either you continue plugging those, too, or you face the avalanche of adenosine when you quit: headaches and unshakable drowiness. Anecdotally, small amounts of caffeine regularly throughout the day (e.g. thermos of green tea or caffeinated mints) seem less likely to trigger this, but I'm curious how low the threshold actually is.