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Model caffeine blood plasma concentrations, hepatic CYP1A2 first-order elimination half-life, adenosine receptor antagonism, and optimal evening cutoff windows to prevent sleep disruption.
30 to 45 minutes post-ingestion.C(t) = C0 × (0.5)^(t / t_half), where t_half = 5.0 Hours (Standard baseline).Circulating Caffeine at Bedtime = Doses × (0.5)^(Hours Elapsed / 5.0) (Target: <25mg).≤ 400 mg/day for healthy adults.The average half-life of caffeine is approximately 5.0 hours (range 3 to 7 hours in healthy adults). The quarter-life is 10 to 12 hours. If you consume a 200mg coffee at 2:00 PM, ~50mg remains active in your brain at midnight.
Caffeine competitively blocks adenosine receptors without eliminating adenosine. As adenosine continues building throughout the day, the eventual hepatic clearance of caffeine allows accumulated adenosine to flood receptors all at once, creating sudden acute exhaustion.
Upon waking, the body naturally clears residual adenosine and experiences a cortisol awakening response. Delaying caffeine 60 to 90 minutes allows natural adenosine clearance first, preventing severe midday rebound fatigue.
CYP1A2 gene variants (fast vs slow metabolizers), oral contraceptives (doubles half-life up to 10 hrs), pregnancy (extends half-life up to 15 hrs), and smoking (accelerates clearance by up to 50%).