# Caffeine: what the research actually says

Caffeine works on your sleep for far longer than it works on your alertness — the reason a personal cutoff time beats a rule of thumb.



## All studies

- **The effect of caffeine on subsequent sleep: a systematic review and meta-analysis** — Gardiner C, Weakley J, Burke LM, et al. (2023), Sleep Medicine Reviews 69:101764. Caffeine reduced total sleep by ~45 min and efficiency by 7%. To avoid effects, a coffee (~107 mg) should be ~8.8 h before bed; a pre-workout dose (~217 mg) ~13 h.

- **Wake up and smell the coffee: caffeine supplementation and exercise performance — an umbrella review of 21 published meta-analyses** — Grgic J, Grgic I, Pickering C, Schoenfeld BJ, Bishop DJ, Pedisic Z (2020), British Journal of Sports Medicine 54(11):681–688. Caffeine is ergogenic for endurance, strength and power — which is why timing (not abstinence) is the lever.

- **Coffee, caffeine, and sleep: a systematic review of epidemiological studies and randomized controlled trials** — Clark I, Landolt HP (2017), Sleep Medicine Reviews 31:70–78. Caffeine prolongs sleep latency, reduces total sleep and deep sleep; sensitivity varies with genetics and habitual use.

- **A review of caffeine's effects on cognitive, physical and occupational performance** — McLellan TM, Caldwell JA, Lieberman HR (2016), Neuroscience & Biobehavioral Reviews 71:294–312. Caffeine restores performance lost to sleep deprivation but does not exceed the rested baseline — no free passes.

- **Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed** — Drake C, Roehrs T, Shambroom J, Roth T (2013), Journal of Clinical Sleep Medicine 9(11):1195–1200. 400 mg caffeine taken even 6 h before bed cut objective sleep by >1 h — while subjects barely noticed.

- **Sustaining executive functions during sleep deprivation: a comparison of caffeine, dextroamphetamine, and modafinil** — Killgore WDS, Kahn-Greene ET, Grugle NL, Killgore DB, Balkin TJ (2009), Sleep 32(2):205–216. Stimulants patch alertness but not higher-order judgement after sleep loss.

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Educational summary of published research, not medical advice. Screening scores are not diagnoses. If a score is elevated or you feel unwell, talk to a clinician.

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