How does caffeine actually keep you awake and alert?

Caffeine works primarily by blocking adenosine receptors in the brain. Adenosine is a chemical that builds up throughout the day as a byproduct of neural activity, and as it accumulates it binds to receptors that slow brain activity and make you feel drowsy. Caffeine's molecular structure is similar enough to adenosine that it fits into those same receptors without activating them, essentially acting as a placeholder that prevents adenosine from doing its job. With adenosine blocked, the brain's natural stimulants like dopamine and norepinephrine can operate more freely, increasing neuron firing and alertness. Caffeine also triggers the pituitary gland to release adrenaline, contributing to that heightened, energized feeling. The effects typically kick in within 30 to 60 minutes and last several hours depending on individual metabolism.