Caffeine half-life is the time it takes your body to eliminate half the caffeine from your bloodstream. The average half-life is 5-6 hours, meaning a 200mg dose at 8am leaves ~100mg at 1pm and ~50mg at 6pm. This is why afternoon coffee still affects your 10pm bedtime — there's still a significant amount circulating. This calculator uses the exponential decay formula: C(t) = C₀ × (0.5)^(t/half-life) where C₀ is initial dose and t is time elapsed.

Caffeine metabolism varies enormously between individuals due to genetics (CYP1A2 enzyme variants), age, medications, and lifestyle factors. Fast metabolizers (the lucky ~50%) process caffeine in about 4 hours — their 2pm latte barely touches their 10pm sleep. Slow metabolizers (also ~50%) may take 7-9 hours, meaning their morning coffee still has significant levels at bedtime. Oral contraceptives slow caffeine metabolism by 50%. Pregnancy extends half-life to 15+ hours. Smokers metabolize caffeine ~50% faster than non-smokers.

Research shows 100mg of caffeine can reduce sleep quality even when consumed 6 hours before bed. A 2013 study in the Journal of Clinical Sleep Medicine found caffeine consumed 6 hours before bedtime reduced sleep by more than 1 hour. The "safe" threshold this calculator uses is <20mg at bedtime — roughly equivalent to a cup of decaf. Caffeine disrupts sleep by blocking adenosine receptors (the sleep pressure chemical) and increasing arousal hormones, even if you don't feel stimulated.

Caffeine content varies widely: Espresso shot: 63mg | Drip coffee (8oz): 95-150mg | Starbucks Grande Coffee: 310mg | Cold brew (12oz): 150-240mg | Red Bull (8.4oz): 80mg | Monster (16oz): 160mg | Black tea (8oz): 47mg | Green tea (8oz): 28mg | Matcha (8oz): 70mg | Dark chocolate (1oz): 12mg. Note that "decaf" coffee still contains 2-15mg. Even this small amount can affect sensitive sleepers with slow metabolism.

Even when caffeine doesn't prevent sleep onset, it disrupts sleep architecture — the sequence of sleep stages. Caffeine reduces slow-wave sleep (the deepest, most restorative stage) even when you don't feel it affecting you. This leads to feeling unrested despite sleeping 7-8 hours. Research by Matthew Walker (Why We Sleep) shows chronic caffeine consumption creates an artificial adenosine "debt" — you feel OK because caffeine blocks the fatigue signal, but the underlying sleep deprivation accumulates.

The calculator's "safe cutoff time" is when you should have your LAST caffeinated drink to have <10mg at bedtime. Practical strategies: (1) Switch to half-caf after noon to maintain the ritual without the full dose. (2) Try L-theanine (found in tea) which smooths the caffeine curve without extending half-life. (3) Move afternoon coffee to before a 20-minute nap — the "coffee nap" technique times caffeine absorption perfectly with wake-up. (4) Track your own patterns — some people are truly unaffected by late caffeine (fast metabolizers with genetic tolerance).