How the caffeine curfew calculator works
The half-life math
Caffeine decays exponentially: level = dose × 2^(−hours / half-life). The average adult half-life is ~5 hours, but individuals run 2–10 (genetics, smoking, pregnancy, oral contraceptives, liver function all shift it). A 200 mg coffee at 3 pm with a 5-hour half-life leaves 100 mg at 8 pm, 50 mg at 11 pm — a full half-shot of espresso in your bloodstream at lights-out, antagonizing the adenosine receptors that build sleep pressure all day.
What "curfew" means
The calculator finds the last-caffeine time that leaves ≤ ~50 mg at bedtime — the level most people sleep through without measurable architecture damage. Sensitive metabolizers (long half-life, or 2-hour sensitivity to a morning cup) should target ≤25 mg; tolerant night owls tolerate more. Calibrate with a week of honest bedtime logs.
Why bedtime caffeine matters even if you "sleep fine"
Caffeine may not stop you falling asleep; it measurably thins deep sleep and fragments the second half of the night — which you experience as waking less restored rather than waking more often. Studies show a bedtime dose reduces deep sleep by ~20% even when subjective sleep-onset is unchanged. The debt compounds quietly.
Common doses for the mg field
- Brewed coffee (8 oz): 80–100 mg · Cold brew (16 oz): 200–300 mg
- Espresso shot: 63 mg · Black tea: 40–50 mg · Green tea: 25–30 mg
- Cola (12 oz): 30–35 mg · Energy drink: 80–300 mg · Pre-workout: 150–350 mg