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Caffeine Half-Life
Caffeine has a half-life: the time your body takes to clear half of what you drank. The average is about five hours, but yours might be two or it might be ten. That gap is why an average drinker's 2 pm coffee is still working at 10 pm, and a slow metaboliser's at midnight. Most people who sleep worse than they'd like are sleeping on caffeine they drank hours earlier and can't feel. Move the timing and you fix it without quitting.
Do ยท Daily Evidence Moderate Chapter Sleep

Caffeine keeps you awake by blocking adenosine, the tiredness signal your brain builds up all day 1. Your liver clears it through one enzyme, CYP1A2, and how fast that runs is mostly genetic 2. An average non-smoker halves a dose in about five hours; a fast metaboliser does it in two, a slow one in ten 3.

Half-life is not clearance. After five hours half your coffee is still on board; being properly clear takes three to five half-lives. The sleep effect outlasts even that, because the blocked receptors stay quiet after the caffeine has gone 4.

Afternoon coffee costs measured sleep, even when you don't feel it. A morning 200 mg still altered the overnight EEG thirteen hours later, long after the blood had cleared 4. Across trials: less deep sleep, slower onset, and habitual drinkers underestimating what the polysomnograph records 5.

You may be a slow metaboliser without knowing. Any of these puts you there:

  • The pill. Combined oral contraceptives roughly double your caffeine half-life 7.
  • Pregnancy. By the third trimester you clear it about three times slower 8.
  • Just quit smoking. Smoking sped clearance up by half; it renormalises over four to six days, which is why coffee suddenly wires you 3.
  • One cup makes you jittery. About a third of people carry an ADORA2A variant that makes caffeine more anxiety-provoking whatever the clearance 9.

Set the cut-off by your category, then hold it.

The loop is self-feeding. The afternoon coffee shortens tonight's sleep; the short sleep makes tomorrow harder; you drink more coffee to cover it. Fatigue and fix share a source you can't tell apart, and years of it become calling yourself a bad sleeper 5.

Give it two weeks. The first sober nights read differently on a tracker within days: faster to fall asleep, more deep sleep, fewer 3 am wakings 4. The felt change lags a week, because habitual drinkers have lowered what they expect from a night. By week two the afternoon coffee is optional.

The fine print โ€” when to skip it, and what people get wrong

Some cases need a smaller dose too. Pregnancy: cap at 200 mg/day, morning-only 10. Fluvoxamine or ciprofloxacin can stretch the half-life past thirty hours, so cut the dose 11. Uncontrolled hypertension or arrhythmia: ask your clinician 12.

"I sleep fine on late coffee": subjective onset is a weak detector; the EEG shows the loss 6. "I'm tolerant now": tolerance builds for the alertness, barely for the sleep damage 13. Decaf isn't zero, at 2โ€“15 mg a cup.

References
  1. 1Bjorness TE, Greene RW (2009). Adenosine and sleep. Current Neuropharmacology. link
  2. 2Thorn CF, Aklillu E, McDonagh EM, Klein TE, Altman RB (2012). PharmGKB summary: caffeine pathway. Pharmacogenetics and Genomics. link
  3. 3Carrillo JA, Benitez J (2000). Clinically significant pharmacokinetic interactions between dietary caffeine and medications. Clinical Pharmacokinetics. link
  4. 4Landolt HP, Werth E, Borbely AA, Dijk DJ (1995). Caffeine intake (200 mg) in the morning affects human sleep and EEG power spectra at night. Brain Research. link
  5. 5Clark I, Landolt HP (2017). Coffee, caffeine, and sleep: a systematic review of epidemiological studies and randomized controlled trials. Sleep Medicine Reviews. link
  6. 6Drake C, Roehrs T, Shambroom J, Roth T (2013). Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. Journal of Clinical Sleep Medicine. link
  7. 7Abernethy DR, Todd EL (1985). Impairment of caffeine clearance by chronic use of low-dose oestrogen-containing oral contraceptives. European Journal of Clinical Pharmacology. link
  8. 8Knutti R, Rothweiler H, Schlatter C (1981). Effect of pregnancy on the pharmacokinetics of caffeine. European Journal of Clinical Pharmacology. link
  9. 9Childs E, Hohoff C, Deckert J, Xu K, Badner J, de Wit H (2008). Association between ADORA2A and DRD2 polymorphisms and caffeine-induced anxiety. Neuropsychopharmacology. link
  10. 10EFSA Panel on Dietetic Products, Nutrition and Allergies (2015). Scientific opinion on the safety of caffeine. EFSA Journal. link
  11. 11Jeppesen U, Loft S, Poulsen HE, Brosen K (1996). A fluvoxamine-caffeine interaction study. Pharmacogenetics. link
  12. 12Robertson D, Frolich JC, Carr RK, Watson JT, Hollifield JW, Shand DG, Oates JA (1978). Effects of caffeine on plasma renin activity, catecholamines and blood pressure. New England Journal of Medicine. link
  13. 13Weibel J, Lin YS, Landolt HP, Garbazza C, Kolodyazhniy V, Kistler J et al. (2020). Caffeine-dependent changes of sleep-wake regulation: evidence for adaptation after repeated intake. Progress in Neuro-Psychopharmacology and Biological Psychiatry. link
Related in the handbook (6)
Undermines
  • โ€” An afternoon coffee masks sleep debt and steals the night's sleep you'd use to repay it.
Related
  • โ€” Like a late coffee, an evening drink quietly degrades the sleep you don't blame it for.
  • โ€” The whole reason for a 2 PM cutoff is caffeine's long half-life โ€” it's still in you at bedtime.
  • โ€” Timing matters both ways โ€” an afternoon nap and a too-late coffee compete for the same window; coffee before a short nap can sharpen the wake-up.
  • โ€” Caffeine is the workhorse nootropic, and getting the timing right is the difference between sharper days and wrecked sleep.
  • โ€” How fast you clear caffeine is largely genetic โ€” the same metaboliser difference these tests read.
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