Reviewed
Reviewed on 2026-09-19 against the evidence rules: every claim carries a grade, every claim and every safety flag carries a source, and every citation was re-resolved against the record it names. The read was delegated by the owner on 2026-09-18 and signed by the Lead on his behalf (D-295) — it is a delegated review, not the owner's own reading of the page. Every claim on it carries an evidence grade and a named weakest link. It is educational and is not medical advice.
Last reviewed
The best-established acute effect in this section, and the clearest cost. It reliably sharpens alertness and reaction time when you are flagging; it reliably disrupts sleep six hours later.
Safety
No data in your group
For children and adolescents, EFSA found the available information insufficient to derive a safe intake at all. That is not a permissive finding; it is an absence of one.
Who should not
In pregnancy the ceiling that has been assessed is 200 mg a day, half the adult figure. Anyone with an arrhythmia or an anxiety disorder should discuss caffeine with a clinician rather than titrate it alone.
Interaction
Fluvoxamine raises caffeine concentrations in the body and caffeine lowers fluvoxamine's renal clearance; several tricyclic antidepressants slow caffeine's own metabolism. If you take an antidepressant, your usual dose is not doing what it does in other people.
Adverse effects
Sleep loss is the reliable adverse effect, and it is invisible from inside: the trial above measured disturbance six hours after the dose in people who had gone to bed normally.
What the evidence says, and how strong it is
A grade belongs to one claim, in one population, for one preparation — never to a compound, a food or a page. Open a grade to see the four links between the evidence and the thing you would actually do, and which of them is doing the least work.
Across a large review of controlled human work, caffeine improves vigilance, attention and reaction time following low vigilance, and repeated dosing maintains physical and cognitive capability in occupations with restricted sleep.
Evidence grade A: Evidence supportsIn whom: Healthy adults, most clearly when vigilance is already low
What form: Caffeine, ordinary dietary doses, single administration
Measured as: Vigilance, attention and reaction time, acutely
Why this grade
- Weakest link
- Study quality — design, size, blinding or replication. Nothing on this claim is thin — it is the best-replicated effect in these twelve dossiers — and the ceiling is that the source is a large review of controlled work rather than one pooled estimate.
- What was actually studied
- Healthy adults, including sleep-restricted occupational groups
- Detail worth knowing
- The review's own summary: "Following low vigilance, attention, reaction time and attention improve, but less consistent effects are observed on memory and higher-order executive function, such as judgment and decision making."
The same review reports that effects on memory and on higher-order executive function — judgement, decision-making — are less consistent than the effects on alertness. Caffeine is not established as making you think better; it is established as making you less sleepy.
Evidence grade C: Early research suggestsIn whom: Healthy, rested adults
What form: Caffeine, ordinary dietary doses
Measured as: Memory and higher-order executive function such as judgement and decision-making
Evidence does not support
Why this grade
- Weakest link
- Outcome — what was measured is not what you would notice. Memory and higher-order executive function are reported as inconsistent across studies, which is what not established looks like from the inside.
- What was actually studied
- Healthy adults, including sleep-restricted occupational groups
- Detail worth knowing
- The review's own summary: "Following low vigilance, attention, reaction time and attention improve, but less consistent effects are observed on memory and higher-order executive function, such as judgment and decision making."
A controlled trial gave 400 mg of caffeine at bedtime, three hours before bed and six hours before bed. All three disturbed sleep significantly against placebo, and the size of the lost sleep at six hours was enough for the authors to recommend stopping caffeine at least six hours before bed.
Evidence grade B: May supportIn whom: Adults sleeping at home
What form: 400 mg caffeine at bedtime, 3 hours before bed, or 6 hours before bed
Measured as: Sleep disturbance, self-reported and measured with a portable monitor
Why this grade
- Weakest link
- Study quality — design, size, blinding or replication. One controlled trial produced the six-hour figure, and nothing has repeated it at that timing.
- What was actually studied
- Adults sleeping at home, monitored with a validated portable sleep monitor
- Detail worth knowing
- 400 mg caffeine at bedtime, 3 hours before and 6 hours before bedtime each disturbed sleep significantly against placebo (p < 0.05 for all).
The European Food Safety Authority concluded that single doses up to 200 mg and habitual intakes up to 400 mg a day do not raise safety concerns for healthy non-pregnant adults, and that up to 200 mg a day in pregnancy does not raise a concern for the fetus.
Evidence grade A: Evidence supportsIn whom: Healthy non-pregnant adults; separately, pregnant women
What form: Caffeine from all sources, single dose and habitual daily intake
Measured as: Intakes that do not raise a safety concern
Why this grade
- Weakest link
- Outcome — what was measured is not what you would notice. The outcome is a regulator's conclusion that stated intakes raise no safety concern, which is the strongest thing of its kind and is still a judgement over a literature rather than a measurement.
- What was actually studied
- Healthy adults, pregnant women, children and adolescents in the EU
- Detail worth knowing
- Single doses up to 200 mg and habitual intakes up to 400 mg/day do not give rise to safety concerns for healthy non-pregnant adults; up to 200 mg/day in pregnancy does not give rise to safety concerns for the fetus; for children and adolescents the information available is insufficient to derive a safe intake.
Everything above, and where it comes from
- McLellan TM, Caldwell JA, Lieberman HR (2016). A review of caffeine's effects on cognitive, physical and occupational performance. Neuroscience & Biobehavioral Reviews 71:294–312
- Drake 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 9(11):1195–1200
- EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) (2015). Scientific Opinion on the safety of caffeine. EFSA Journal 13(5):4102
- Truong J, Abu-Suriya N, Tory D, et al. (2025). An Exploration of the Interplay Between Caffeine and Antidepressants Through the Lens of Pharmacokinetics and Pharmacodynamics. European Journal of Drug Metabolism and Pharmacokinetics 50(1):1–15
The one thing worth doing
Work out your usual bedtime, subtract six hours, and make that your last coffee. It costs nothing, it is the best-evidenced change on this page, and it belongs to Recovery rather than to Fuel.
Regulatory note — this note has not yet been reviewed by a lawyer. In the United States, supplements are sold under DSHEA: a structure/function claim is not reviewed by the FDA before it appears, and the label must carry the words "This statement has not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease." In the EU and the UK, only health claims on the authorised register may be used in advertising, and a food or supplement advertisement may not say a product prevents, treats or cures a disease. Whether an evidence page that carried a purchase link would itself count as advertising under those rules is an open question that counsel, not this page, will answer; there are no purchase links here. Nothing on this page is medical advice.