It's a myth that caffeine gives you energy. In fact, through a kind of clever biological trickery, caffeine fools your body into thinking that it isn't tired.
Business Insider reports that the longer a person is awake, the more a compound called adenosine builds up in their brain. Adenosine attaches to receptors in the brain, slowing down synaptic connections and making you feel tired.
Molecularly, caffeine and adenosine are similar enough to fool the brain's receptors. Caffeine binds to those receptors, preventing adenosine from establishing a toe-hold and staving off that feeling of tiredness.
After a time, the brain lets go of those caffeine molecules, allowing the adenosine to flow back in, and allowing the body to feel all of the accumulated tiredness, which can lead to the post-caffeine crash.
Also, over periods of prolonged use, the brain's receptors bind to the caffeine molecules less efficiently, meaning that coffee and sodas work less and less effectively the longer you stay awake and drink them.
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