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Caffeine doesn't create energy in the way calories do.
Instead, it primarily works by blocking receptors for adenosine, a chemical involved in increasing feelings of sleepiness as you've been awake longer.
Because those receptors are temporarily blocked, you can feel more alert even though your body's underlying need for sleep hasn't disappeared.
When caffeine wears off, the accumulated sleep pressure can still be there.
Coffee can change how awake you feel without replacing the sleep your body actually needs.
© Sleep Foundation