Subjective alertness
Human researchIncreased
Participants reported greater alertness after caffeine in the cited crossover trial. [1]
- Population
- 27 healthy adult volunteers
- Exposure context
- 50 mg caffeine; acute crossover comparison with placebo
THE SUBSTANCE LIBRARYSourced draft
CaffeineWakefulness, with a long tail.
A familiar stimulant with an unusually rich evidence base. Its effects depend on timing, tolerance and how quickly your body clears it.
01
Overview
Caffeine is a methylxanthine found in coffee and tea and added to some foods and medicines. Blocking adenosine signaling can increase alertness, while residual exposure can interfere with sleep. The evidence below separates measured outcomes from the experience of feeling more awake.
Sourced draft · editorial review pending
Selected evidence covers alertness, sleep disruption and pharmacokinetics. Claims have not undergone formal evidence grading or independent editorial review.
Assessment belongs to each claim and outcome. An article’s editorial status is not a grade of a substance’s safety or efficacy.How it works
Adenosine receptor antagonism
Caffeine interferes with adenosine signaling. Feeling less sleepy does not mean the underlying need for sleep has disappeared. [2]
02
Subjective effects
Descriptions of experience in their reported context. Direction does not imply benefit, and these observations do not define a universal intensity score.
Anxiety / jitteriness
Human researchVariable
Adverse experiences depend on exposure and sensitivity; vulnerable populations need separate consideration. [2]
- Population
- Populations represented in a narrative safety review
- Exposure context
- Varied caffeine intakes and exposure histories
03
Measured outcomes
What the research measured, for whom, and under which exposure. Findings remain attached to the study’s task or clinical endpoint.
Attention-task accuracy
Human researchIncreased
Accuracy improved on a specific switching task; this does not quantify a general cognitive boost. [1]
- Population
- 27 healthy adult volunteers
- Exposure context
- 50 mg caffeine; assessment 90 minutes after administration
- Measure / instrument
- Attention-switching task accuracy
- Magnitude
- Not quantified in this summary
Sleep continuity
Human researchDecreased
The sleep trial found disruption even when the studied exposure preceded bedtime by several hours. [5]
- Population
- Adults in a placebo-controlled home sleep study
- Exposure context
- 400 mg caffeine at bedtime or 3 or 6 hours before it
- Measure / instrument
- Self-report and validated portable sleep monitoring
- Magnitude
- Not quantified in this summary
04
Doses & routes
Published exposure records, distinguished by source category. These describe study or reference context and are not personal dosing recommendations.
Attention study
Research exposure50 mg [1]
- Ingredient / form
- Caffeine · Study treatment; exact preparation not assessed
- Route
- Oral
- Frequency
- Single exposure per crossover session
- Duration
- Acute assessment at 60 and 90 minutes
- Population
- 27 healthy adult volunteers
- Purpose
- Compare attention and mood with placebo and a caffeine–theanine combination
Studied alone and with 100 mg L-theanine. A trial exposure, not a recommended personal dose.
Sleep-disruption study
Research exposure400 mg [5]
- Ingredient / form
- Caffeine · Study treatment; exact preparation not assessed
- Route
- Oral
- Frequency
- Single exposure on each test night
- Duration
- At bedtime, or 3 or 6 hours before bedtime; sleep followed that night
- Population
- Adults in a placebo-controlled sleep study
- Purpose
- Assess timing-dependent sleep disruption
Given at bedtime or 3 or 6 hours before it. Sleep was disrupted at every tested timing; this is not a suggested dose.
05
Pharmacokinetics
Absorption, metabolism and elimination depend on the analyte, route, formulation, physiology and other exposures.
Elimination half-life · Caffeine3–7 hours [2]
OnsetSubjective onset varies [2]
PeakSerum peak around 2 hours in the cited review; formulation and study matter [2]
DurationSleep effects may outlast perceived stimulation [2]
A commonly reported adult range, not an individual prediction. Pregnancy, medications, smoking and liver function can shift it substantially. [2]
ILLUSTRATIVE ELIMINATION MODEL
Single exposureCaffeine remaining over time
The modeled analyte is Caffeine. The curve begins after absorption and distribution; it does not predict onset, felt effects, or an individual’s response.
Route: Oral. Formulation: Ingested caffeine; preparations vary across reviewed studies. Population: Adults represented in the narrative review.
A commonly reported adult range, not an individual prediction. Pregnancy, medications, smoking and liver function can shift it substantially. Source & context
Reported range: 3–7 hours.
The midpoint is the initial illustrative choice within the reported range. It is not a measured average or a prediction.
Caffeine remaining (%)
After 5 h50% remainsAfter 10 h25% remainsAfter 25 h3.125% remains
Model: fraction remaining = 2−time / half-life. Assumes a single exposure, instantaneous distribution, and a constant half-life for Caffeine. Formation of metabolites, repeated exposure, and interactions are not modeled. Source & context
Sourced elimination observations
Caffeine
3–7 hoursReported range [2]
- Route / formulation
- Oral · Ingested caffeine; preparations vary across reviewed studies
- Population
- Adults represented in the narrative review
A commonly reported adult range, not an individual prediction. Pregnancy, medications, smoking and liver function can shift it substantially.
- Bioavailability
- Rapid, near-complete intestinal absorption [2]
- Metabolism / metabolites
- Primarily hepatic CYP1A2; paraxanthine is the major metabolite [2]
What changes the picture?
Tobacco smoke
Faster clearance during smokingSmoking induces CYP1A2. In a small study of heavy smokers, caffeine clearance fell after cessation. This is not evidence that nicotine itself accelerates caffeine clearance. [4]
Related observationCYP1A2 activity
Exposure varies between peopleGenetics, medicines and physiological context affect clearance. A genotype alone cannot determine a precise personal half-life. [3]
Related observationOral contraceptives
Slower clearance in reviewed dataThe systematic dataset found longer elimination half-life with oral contraceptive use; formulation and individual context still matter. [3]
Related observation06
Safety & uncertainty
Adverse effects, interactions, tolerance and withdrawal in the cited contexts. This section is not an exhaustive interaction checker.
Sleep is part of the outcome
A short-lived improvement in alertness can coexist with later sleep disruption. Half-life is not the same as duration of a noticeable effect. [5]
Changing smoking status changes exposure
Caffeine clearance can decrease after stopping smoking; previous intake may then produce different exposure. [4]
Population matters
Pregnancy, childhood, heart conditions and some psychiatric conditions change the risk context. Adult study exposures are not universal guidance. [2]
07
Research & sources
6The source, the finding and its limitations. Funding information is reported where curated; an unassessed disclosure does not mean a study had no commercial funding.
- 01Randomized crossover trial · 2008
The combined effects of L-theanine and caffeine on cognitive performance and mood
Caffeine with L-theanine improved some attention-task outcomes in 27 healthy volunteers.
Limitations Small, short-term study; selected laboratory tasks do not establish everyday productivity or long-term benefit. Authors were affiliated with Unilever.
Funding / disclosures Funding not assessed; author affiliations include Unilever.
- 02Narrative review · 2017
The Safety of Ingested Caffeine: A Comprehensive Review
Reviews absorption, adenosine signaling, adult elimination and population-specific safety concerns.
Limitations A broad review; its typical kinetic values cannot predict an individual's response.
Funding / disclosures Not assessed; consult the original disclosure.
- 03Systematic pharmacokinetic analysis · 2022
Pharmacokinetics of Caffeine: A Systematic Analysis of Reported Data
Integrates human kinetic data and examines smoking, contraceptives, medicines and disease as modifiers.
Limitations The underlying studies use different populations and methods; the result is not a personal clearance calculator.
Funding / disclosures Not assessed; consult the original disclosure.
- 04Human pharmacokinetic study · 2004
Time response of cytochrome P450 1A2 activity on cessation of heavy smoking
Caffeine clearance decreased after cessation in 12 heavy smokers.
Limitations Small selected sample. The reported half-life of enzyme activity change must not be confused with caffeine elimination half-life.
Funding / disclosures Not assessed; consult the original disclosure.
- 05Randomized controlled trial · 2013
Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed
The tested caffeine exposure disrupted sleep at each tested timing.
Limitations A single fixed exposure and a small sample do not define a universal bedtime cutoff.
Funding / disclosures Not assessed; consult the original disclosure.
- 06Chemical database · 2026
Caffeine: compound record and molecular structure
Source for molecular identity, formula, molecular weight and the structure diagram.
Limitations A compound record does not establish a product's purity, identity or clinical benefit.
Funding / disclosures Public database maintained by NCBI.
08
Connected claims
Each relationship retains its participants, roles and context. Shared membership does not imply that substances should be combined.
Caffeine antagonizes adenosine receptors.
- Caffeineagent
- Adenosine receptorstarget
- Adenosineendogenous-ligand
Human pharmacology; receptor activity does not erase physiological sleep need.
Limitation Mechanism alone does not establish the size or duration of a clinical effect.
Supporting sources [2]
Evidence strength: not formally assessed.
Tobacco-smoke exposure induces CYP1A2 activity; stopping smoking reduced caffeine clearance in the cited study.
- Caffeinesubstrate
- CYP1A2metabolizing-enzyme
- Tobacco-smoke exposureexposure-context
Heavy smokers undergoing cessation. Tobacco smoke is a metabolic exposure, not a caffeine route.
Limitation Enzyme activity decline after cessation is not caffeine elimination half-life.
Supporting sources [4]
Evidence strength: not formally assessed.
The tested caffeine exposure disrupted subsequent sleep.
400 mg administered at bedtime or 3 or 6 hours before it.
Limitation One fixed exposure cannot establish a universal sleep cutoff.
Supporting sources [5]
Evidence strength: not formally assessed.
A caffeine–L-theanine combination improved selected attention task results.
- Caffeineco-exposure
- L-theanineco-exposure
- Attentionmeasured-outcome
27 healthy volunteers; acute crossover comparison with placebo.
Limitation A co-study does not establish long-term benefit or synergy.
Supporting sources [1]
Evidence strength: not formally assessed.
09
Legal context
Not assessed. Absence of a legal record is not a statement of legal status.
10
Editorial history
This article is a sourced draft. Editorial review has not been recorded. Content date: Sep 29, 2026.