Tea
Caffeine: what it does and how much is in things
A stimulant with reasonably well-understood effects, present in wildly variable amounts depending on preparation.
Caffeine is the most widely consumed psychoactive substance in the world and it is present in coffee, tea, cacao, and a few other plants.
How it works
Adenosine accumulates in the brain during waking hours and promotes sleepiness by binding to its receptors.
Caffeine is structurally similar enough to occupy those receptors without activating them, blocking adenosine's signal.
The tiredness is not removed; it is masked. When the caffeine clears, the accumulated adenosine binds, which is part of why a crash follows.
How long it lasts
The half-life in most adults is roughly three to six hours, meaning half the dose remains after that period.
Considerable individual variation exists, driven substantially by genetics — variation in the liver enzyme that metabolises caffeine produces fast and slow metabolisers with meaningfully different experiences of the same dose.
Other factors: pregnancy substantially extends the half-life; some medications including certain oral contraceptives slow metabolism; smoking accelerates it.
The practical implication: a coffee at four in the afternoon still has a meaningful quantity of caffeine in your system at bedtime, whether or not you notice it.
Sleep
The effect people most often underestimate.
Studies have found that caffeine consumed several hours before bed measurably disrupts sleep architecture — reducing deep sleep and total sleep time — even in people who report falling asleep normally.
Which is worth knowing: the absence of a subjective effect is not evidence of no effect.
A common recommendation is to stop caffeine eight to ten hours before bed, which for most people means early afternoon.
People who sleep poorly and drink coffee late are frequently surprised by what a two-week experiment reveals.
Rough amounts
Highly variable, and these are typical ranges rather than fixed values.
Filter coffee, a two hundred forty millilitre cup: roughly eighty to one hundred forty milligrams.
Espresso, a single shot: roughly sixty to eighty milligrams. A double, correspondingly more.
Note that espresso has more per millilitre and less per serving than filter, which is why the assumption that espresso is the strongest is misleading.
Instant coffee: roughly sixty to eighty milligrams.
Black tea: roughly forty to seventy milligrams.
Green tea: roughly twenty to forty-five milligrams.
Matcha, where the whole leaf is consumed: roughly sixty to eighty milligrams per serving.
Decaf coffee: typically two to seven milligrams, not zero.
Energy drinks: extremely variable, frequently eighty to two hundred milligrams per can.
Variation within each category is large. Brewing strength, bean and leaf quantity, and serving size matter more than the category.
The dose question
Health authorities in several jurisdictions have suggested that up to around four hundred milligrams per day is not associated with adverse effects for most healthy adults — roughly three to four cups of coffee.
Pregnancy guidance is substantially lower, commonly around two hundred milligrams.
Individual sensitivity varies enormously. Some people are uncomfortable at one cup; others tolerate considerably more.
Anyone with a heart condition, anxiety disorder, or on medication that interacts with caffeine should take medical advice rather than population guidance.
Tolerance and withdrawal
Regular consumption produces tolerance to some effects, particularly the subjective alertness boost, within days to weeks.
Which means much of habitual coffee drinking is restoring baseline rather than exceeding it.
Withdrawal is real and well documented: headache, fatigue, irritability and difficulty concentrating, typically beginning twelve to twenty-four hours after the last dose and lasting several days.
Tapering rather than stopping abruptly avoids most of it.
Theanine, and why tea feels different
Tea contains theanine, an amino acid that crosses into the brain and is associated with a calmer subjective state.
Many people report that the alertness from tea feels smoother and more sustained than from coffee at comparable caffeine doses, and theanine is the usual explanation.
The research on theanine and attention is developing rather than settled. The subjective reports are consistent enough to be worth taking seriously.
Shade-grown teas — matcha and gyokuro particularly — contain substantially more theanine, which is a deliberate consequence of the shading.
Practical adjustments
If you sleep badly, move your last caffeine earlier before reducing the amount.
If you get jittery, spread intake across the day rather than concentrating it.
If you crash mid-afternoon, that is frequently the first dose wearing off rather than a need for more.
If you want to reduce, blend decaf with regular in increasing proportions over a couple of weeks. It works and avoids withdrawal.
General information about caffeine, not medical advice. Individual responses and appropriate intake vary. Consult a clinician about your own circumstances, particularly during pregnancy or with any cardiac or anxiety condition.
