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How does caffeine increase alertness in people who drink coffee?

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Paper bags filled with roasted coffee beans, ground coffee, and instant coffee granules arranged closely together. — coffee Roasted coffee beans, ground coffee, and instant coffee in paper bags.

Caffeine increases alertness by acting as a stimulant on the central nervous system through its interaction with adenosine.12

Adenosine is a byproduct of cell metabolism that naturally accumulates in the brain and spinal column throughout the day. When adenosine binds to its specific receptors, it signals a need for sleep, creating what is known as sleep pressure.12

Because caffeine is structurally similar to adenosine, it can bind to these same receptors. However, unlike adenosine, caffeine does not trigger sleep signaling. By occupying these receptors, caffeine effectively blocks the body from receiving the signal that it is time to rest, thereby delaying sleep and promoting a state of increased alertness and decreased fatigue. This mechanism is why many people rely on coffee as part of their daily routine to feel more energized.12