A caffeine crash is what happens when caffeine is dislodged from the adenosine receptor in your brain. As Dr. Matthew Walker explained on my podcast, caffeine has a half-life of about five to six hours. When you consume caffeine, it blocks adenosine, a sleep-inducing molecule, from communicating to your brain, making you feel less tired. However, as the caffeine starts to leave your system, you're hit with the same levels of adenosine that were present before you had the caffeine, plus all the adenosine that's been building up while the caffeine was in your system. This results in a sudden wave of tiredness, or what we call a caffeine crash.