How the curve is made
The instrument draws one curve: the caffeine in your body across a day. This is everything behind it, in plain words, with every number tied to the source it came from.
What the curve claims
A model of you, not a verdict on you
The curve is the milligrams of caffeine in the body of a typical adult, hour by hour: what each drink adds, how it fades, and what is still there at bedtime. It is a model, not a measurement. It is not a blood level, it has never met your liver, and it does not tell you whether you will sleep.
It is an educational model of a typical body, and not advice for pregnancy, illness or medication. The numbers below are the ones the model and the drink table use, each with its source.
How fast does caffeine kick in?
Kicks in right after the meeting
Caffeine reaches its highest level in the blood 30 to 120 minutes after it is swallowed, not at the first sip. 22
Caffeine's stimulating effects may begin 15 to 30 minutes after it is swallowed and last a number of hours. 23
Caffeine taken by mouth is absorbed rapidly and completely. 2223
So the curve starts at zero at the first sip and climbs, rather than jumping. On the instrument, you choose where in 30–120 min your peak lands.
Once absorbed, caffeine freely crosses the blood-brain barrier, the placenta and the blood-testis barrier; it readily passes into the fetus. 22
Caffeine spreads throughout body water. EFSA gives its volume of distribution as 0.671 L/kg body weight; an Institute of Medicine review gives 0.7 L/kg, a value it says suggests caffeine is hydrophilic and distributes freely into the water inside cells. 2224
How long does caffeine stay in you?
Half of it is still in there
In healthy adults, caffeine's half-life, the time the body takes to eliminate half of it, averages about 4 hours, with a range of about 2 to 8 hours. It varies with factors such as age, body weight, pregnancy, medication and liver health. 2223
That spread is why half-life is a control on the instrument and not a hidden constant. Move it and watch how much of the morning is still there at night.
If caffeine leaves the body with a constant half-life of h hours, the share of a dose still present t hours later is 0.5^(t/h). At a 5-hour half-life, a morning dose is down to a quarter of itself 10 hours later and an eighth 15 hours later; at a 4-hour half-life, about 18% is left after 10 hours. This counts elimination only and ignores the time the dose takes to be absorbed. 23
Caffeine is cleared mainly in the liver by the enzyme CYP1A2, which accounts for about 95% of its clearance. The main route, taking 70 to 80% of caffeine, turns it into paraxanthine; theophylline and theobromine are the other primary metabolites. 2224
Caffeine kinetics have been reported to be linear (dose-independent) at doses up to 10 mg/kg body weight. A later study claimed non-linearity starting at 500 mg, about 7.1 mg/kg body weight. 22
Some things change the half-life a lot.
Smoking speeds caffeine clearance. In a study of 786 people, clearance rose with the number of cigarettes smoked a day, from 1.22 times that of non-smokers at 1 to 5 a day to 1.72 times at more than 20 a day. 22
Oral contraceptives slow caffeine clearance. In a study of 786 people, oral contraceptive use cut clearance to 0.72 times that of non-users. In a small study of eight people, the two taking oral contraceptives had caffeine half-lives of about 15.5 hours, against about 5.6 hours in the others. An Institute of Medicine review says oral contraceptive use can double caffeine half-life. 2224
Pregnancy lengthens caffeine's half-life because CYP1A2 activity falls. In 15 pregnant women, half-life rose to 6 to 16 hours and returned to 2 to 8 hours within 4 to 15 weeks after delivery; for non-smoking women at the end of pregnancy, other reports give 11.5 and 18 hours, and EFSA puts the end-of-pregnancy half-life at three to four times the non-pregnant value. Caffeine readily crosses the placenta, and neither the fetus nor the placenta can metabolise it. 22
Genes get a lot of credit for the difference between people. The evidence gives them less.
Some authors call people with the AA form of the CYP1A2 variant rs762551 (also known as -163C>A or CYP1A2*1F) 'fast' caffeine metabolisers and those with AC or CC 'slow', but the variant changes how far the enzyme can be induced rather than splitting people cleanly. In two studies, AA carriers had higher CYP1A2 activity only among smokers, with no difference between genotypes in non-smokers; in a third, drinking more than two cups of coffee a day was linked to higher activity only in AA non-smokers. In 786 people, caffeine clearance showed no relevant functional CYP1A2 polymorphism once coffee intake, smoking, oral contraceptives and sex were accounted for. 22
How much caffeine is too much?
Yes, it’s my sixth. No, we’re not discussing it
EFSA concludes that caffeine intakes up to 400 mg a day from all sources, spread through the day, do not raise safety concerns for healthy adults, except pregnant women. FDA cites 400 mg a day for most adults as an amount not generally associated with negative effects, and notes wide variation in how sensitive people are to caffeine and how fast they eliminate it. 225
EFSA concludes that single doses of caffeine up to 200 mg, about 3 mg/kg body weight for a 70 kg adult, from all sources do not raise safety concerns for healthy non-pregnant adults. The same amount does not raise safety concerns when taken less than two hours before intense exercise in normal environmental conditions, but no studies are available in pregnant women or in middle-aged or elderly people undertaking intense exercise. Given caffeine's different kinetics in pregnancy, EFSA says these single-dose levels may not apply to pregnant women who exercise. 22
EFSA concludes that for pregnant women, caffeine intakes up to 200 mg a day from all sources, spread through the day, do not raise safety concerns for the fetus. The line rests on prospective cohort studies showing a dose-dependent link between caffeine intake in pregnancy and the risk of birth-weight outcomes such as fetal growth retardation and being small for gestational age. 22
EFSA found the evidence for children and adolescents insufficient to derive a safe caffeine intake. It uses the adult single-dose level of no concern, 3 mg/kg body weight, as the basis for both single doses and daily intake in this group. As in adults, about 1.4 mg/kg may lengthen the time to fall asleep and shorten sleep in some children and adolescents, particularly close to bedtime. 22
FDA estimates that toxic effects, such as seizures, can be observed with rapid consumption of around 1,200 mg of caffeine, less than half a teaspoon of pure caffeine. 5
These are reference lines, not targets, and not a personal diagnosis. The instrument draws none of them as a danger zone.
Does caffeine in the afternoon affect sleep?
Tonight’s sleep was decided at 3 pm
A single 100 mg dose of caffeine, about 1.4 mg/kg body weight for a 70 kg adult, may increase the time it takes to fall asleep and reduce sleep duration in some adults, particularly when taken close to bedtime. 2223
In Drake et al. 2013, 12 healthy normal sleepers took 400 mg of caffeine as a pill at bedtime, 3 hours before or 6 hours before their usual bedtime, or placebo, on separate nights at home, and sleep was disturbed versus placebo at each time. On a home sleep monitor, total sleep fell significantly, by 1.1 to 1.2 hours versus placebo, at each time, and the discussion says that even at 6 hours caffeine reduced sleep by more than 1 hour. In the sleep diaries (Table 2), the 6-hour dose cut self-reported total sleep by 41 minutes, a result that only approached significance (p = 0.08). The study was small and used a single-channel EEG headband, not polysomnography. 2526
Both readings belong here: the monitor’s 1.1–1.2 h less sleep at every dose time, and the diaries’ 41 min for the earliest dose, which only approached significance. It is one small study, and the curve does not pretend otherwise.
That is why the instrument shows a number and a target line you set yourself, and no red zone telling you that your night is ruined.
What caffeine does, and what stopping does
Adenosine can wait
Caffeine's effects come mainly from blocking adenosine receptors: of the four types, it blocks A1 and A2A, which are found in the central nervous system. It also acts, at least in part, by facilitating dopamine D2 receptor transmission, a mechanism that appears substantially different from that of stimulants such as cocaine and amphetamine. 22
Tolerance develops to some but not all of caffeine's effects, and how much develops varies widely between people. Tolerance to its effects on blood pressure and heart rate usually develops within a couple of days. Whether tolerance explains why people differ in how coffee affects their sleep is uncertain; some authors put those differences down to variation in sensitivity between and within people instead. 22
When regular caffeine use stops, withdrawal symptoms typically begin 12 to 24 hours later, peak at 20 to 51 hours and last 2 to 9 days. Validated symptoms include headache, fatigue, decreased energy, decreased alertness, drowsiness, depressed mood, difficulty concentrating and irritability. Stopping doses as low as 100 mg a day produced symptoms. 2722
The equations
I did the maths. You can keep ignoring it
The model is a one-compartment model with first-order absorption and first-order elimination. All of the caffeine is taken as absorbed, as the absorption section says. Each drink gets its own curve, and the curve on the chart is their sum.
ke = ln 2 / t½
How fast caffeine leaves, from the half-life.
tmax = ln(ka / ke) / (ka − ke), solved for ka > ke
How fast it arrives, from the time to peak.
A(t) = D · ka / (ka − ke) · (e−keΔt − e−kaΔt)
What is left of one drink of D mg, Δt hours after the sip. Before the sip it is zero. Bioavailability is taken as 1: all of it is absorbed 2223
Total(t) = Σ Ai(t)
The curve on the chart: every drink’s amount, added up.
These are settings, not findings. The instrument starts at a 5 h half-life, which you can set from 2 to 16 h, and a 45 min time to peak, from 30 to 120 min.
Sources
- Development of a Caffeine Content Table for Foods, Drinks, Medications and Supplements Typically Consumed by the Brazilian Population (BraCaffT)
- What is caffeine?
- Coca-Cola (US), 12 fluid ounce can: SmartLabel nutrition facts
- Coca-Cola Zero Sugar: Discover Sugar-Free Soda (US)
- Spilling the Beans: How Much Caffeine is Too Much?
- Monster Energy Original Green
- Pepsi - 12 fl oz (PepsiCo Product Facts)
- Pepsi Zero Sugar - 12 fl oz (PepsiCo Product Facts)
- How much caffeine is in a can of Red Bull Energy Drink?
- Red Bull Energy Drink: Ingredients
- Beverages, coffee, brewed, prepared with tap water
- Beverages, coffee, brewed, espresso, restaurant-prepared
- Beverages, tea, green, brewed, regular
- Beverages, tea, black, brewed, prepared with tap water
- Milk, whole, 3.25% milkfat, with added vitamin D
- Milk, reduced fat, fluid, 2% milkfat, with added vitamin A and vitamin D
- Milk, nonfat, fluid, with added vitamin A and vitamin D (fat free or skim)
- Oat milk, unsweetened, plain, refrigerated
- Soy milk, unsweetened, plain, shelf stable
- Almond milk, unsweetened, plain, refrigerated
- Quantitative Analysis of Caffeine in Roasted Coffee: A Comparison of Brewing Methods
- Scientific Opinion on the safety of caffeine
- EFSA explains risk assessment: Caffeine
- Pharmacology of Caffeine (chapter 2 of Caffeine for the Sustainment of Mental Task Performance: Formulations for Military Operations)
- Caffeine Effects on Sleep Taken 0, 3, or 6 Hours before Going to Bed
- Late afternoon and early evening caffeine can disrupt sleep at night
- A critical review of caffeine withdrawal: empirical validation of symptoms and signs, incidence, severity, and associated features
- Excedrin Extra Strength: Drug Facts (DailyMed label)
- Excedrin Migraine: Drug Facts (DailyMed label)
- Excedrin Tension Headache: Drug Facts (DailyMed label)
- Anacin (DailyMed label)
- Midol Complete (DailyMed label)
- NoDoz Alertness Aid Maximum Strength (DailyMed label)
- Vivarin (DailyMed label)
- BC Original (DailyMed label)
- Goodys Extra Strength (DailyMed label)
- Fioricet® Capsules (DailyMed label)
- Ergotamine Tartrate and Caffeine Tablets, USP 1 mg/100 mg (DailyMed label)
- Production, Supply and Distribution: Coffee (psd_coffee_csv.zip)
- World Population Prospects 2024: Total Population by Sex (WPP2024_TotalPopulationBySex.csv)