Caffeine and adenosine are very similar in chemical structure. In short, caffeine is used to increase levels of adrenaline (epinephrine) and adenosine causes drowsiness. Adenosine is a neurotransmitter that binds to receptor sites, slows neural activity, and later causes us to fall asleep. Caffeine, a psychoactive substance, interrupts this process by removing adenosine, taking its place, and blocking further accumulation. Adenosine, at that point, is no longer available to slow neural drive, thus creating the opposite effect: central nervous system stimulation. This is beneficial when adenosine signals a “rest and digest” (parasympathetic) response but we prefer a “fight or flight” (sympathetic) response. The effects of caffeine peaks after ~1 hour of ingestion and has a half-life of ~6 hours. This means, if you consume 200 milligrams of caffeine at 6am, ~100 milligrams will still be in your system by 12pm.