Caffeine

What it does

What changes, what doesn't, and what too much feels like.

At a normal dose

• Alert: More alert and vigilant, with sharper attention.
• Faster: Quicker reaction times.
• Stronger: Above about 200 mg, better endurance, strength and sprinting.
• Not smarter: Its effects on memory, judgement and decision-making are less consistent.
• Keeps you going: When sleep is short, repeated doses help people keep performing.

Too much

• A fast or pounding heart, and raised blood pressure.
• Jitters, nervousness, anxiety and irritability.
• Nausea, an upset stomach, headache.
• Trouble sleeping.

Why the same coffee hits people differently

One liver enzyme, CYP1A2, breaks down 95% of caffeine, and common genetic variants make how fast people clear it vary a lot. Age, hormones, liver health, smoking and diet change it too, and variants in the adenosine receptor change how much caffeine disturbs sleep or raises anxiety.

What's happening in the brain

A tiredness signal blocked, a reward circuit primed, and less blood to the brain.

While you're awake, adenosine slowly accumulates. In cats it rose steadily in the basal forebrain the longer they stayed up, fell during sleep, and raising it artificially brought on the sleepiness of a long day. It's part of how your brain keeps track of how long you've been awake.

Caffeine fits adenosine's receptors without switching them on, so adenosine can't act: its calming, braking effect is held back. Meanwhile adenosine keeps building the longer you're awake, so caffeine hides the tiredness rather than taking it away.

The alertness circuit

On a PET scan, 300 mg of caffeine made dopamine receptors more available in the putamen and ventral striatum, and the more it did in the ventral striatum, the more alert people felt. It didn't seem to release more dopamine itself.

Caffeine also narrows the brain's blood vessels: 250 mg cut blood flow to the brain by about 27%, and heavy users adapted only partly.

How much is in what

The limits, and how fast you can reach them.

• 400 mg a day, for healthy adults: Health Canada's recommended maximum.
• 2.5 mg/kg a day, under 18: So about 125 mg for someone who weighs 50 kg.
• 300 mg a day, in pregnancy: And when breastfeeding or planning a pregnancy.

Roughly how much is in…

• Brewed coffee: About 135 mg in a 237 ml cup.
• Energy drinks: About 54 to 328 mg in 16 oz (473 ml).
• Black or green tea: 30 to 50 mg a cup.
• Cola: 36 to 46 mg a can.
• Decaf coffee: Not zero: 2 to 15 mg a cup.

Hidden caffeine

It's added to some protein bars, gum, supplements and over-the-counter medicines too, and restaurants don't have to say how much is in their drinks.

Sleep and withdrawal

The cost you don't feel, and the one you do.

In a sleep study, 400 mg of caffeine disrupted sleep whether it was taken at bedtime, three hours before, or six. Even at six hours it cut more than an hour of sleep on a monitor, yet people's own sleep diaries showed no clear difference: they lost the sleep without knowing it. The researchers advise no substantial caffeine for at least six hours before bed.

If you stop suddenly

• 12–24 hours, It starts: Headache, tiredness, low energy, drowsiness.
• 20–51 hours, The worst of it: Low mood, irritability, trouble concentrating, feeling foggy; sometimes flu-like symptoms.
• 2–9 days, It passes: Withdrawal fades within days.

About half of people in withdrawal experiments got a headache, and about 13% were affected enough to struggle with daily life. Stopping as little as 100 mg a day, roughly one cup of coffee, can do it. It isn't dangerous, and cutting back gradually makes it easier.

When it's dangerous

Coffee is rarely the problem. Concentrated caffeine can be.

Pure and powdered caffeine

Taking about 1,200 mg quickly, less than half a teaspoon of pure caffeine, can cause seizures, and highly concentrated products have killed people. A measuring mistake with powder is all it takes.

Be careful

• Energy drinks for children and teens: medical experts advise against them.
• Several energy drinks in a row: one can hold up to 328 mg.
• Caffeine late in the day, if sleep matters: even six hours before bed it costs sleep.

If someone has a racing or pounding heart, a seizure, or bad nausea after a lot of caffeine, get medical help.

Check what you know

Four everyday situations.

• You have a large coffee at 4 p.m. and go to bed at 10. You fall asleep fine. Did the coffee cost you sleep? Probably, even if it didn't feel like it. Caffeine six hours before bed cut more than an hour of sleep on a monitor, but not clearly in people's own diaries.
• It's Saturday. You skipped your usual morning coffee and by evening you have a pounding headache and feel foggy. What's most likely going on? Caffeine withdrawal, which can start 12–24 hours after your last dose. Headache and fogginess are core withdrawal symptoms, starting 12–24 hours after stopping.
• Your 15-year-old cousin weighs 50 kg and drinks two large energy drinks before a game. How does that compare with Health Canada's guidance? It's likely well over: the limit for him is about 125 mg, and one can can hold up to 328 mg. Under 18 it's 2.5 mg per kg a day, and energy drinks hold 54–328 mg per 16 oz.
• Before an exam, a friend says three coffees will make her smarter. What does the research say? It'll make her more alert and quicker, but it won't reliably improve memory or judgement — and too much brings jitters and anxiety. Caffeine improves alertness, attention and reaction time, with less consistent effects on memory and decision-making; too much brings anxiety and jitters.