Caffeine: how much is safe, and when to stop for the day

How much is in what you drink

Caffeine content varies widely with the bean, the method and the size of the cup. Approximate figures:

• Espresso, single: 60 to 80 mg.
• Filter coffee, 200 ml: about 90 mg.
• A large coffee-shop coffee, 400 to 500 ml: 200 to 300 mg.
• Black tea, 220 ml: about 50 mg.
• Green tea: 30 to 40 mg.
• A 250 ml energy drink: 80 mg.
• A 330 ml cola: about 35 mg.
• Dark chocolate, 50 g: 25 to 40 mg.

Arabic coffee, served in small cups, contains little per cup; the total depends on how many are accepted in the course of a morning of meetings.

What the safety assessment says

The European Food Safety Authority reviewed the evidence in 2015 and concluded the following for healthy adults.

• Up to 400 mg a day from all sources does not give rise to safety concerns.
• Single doses of up to 200 mg, about 3 mg per kilogram of body weight, do not give rise to safety concerns, including when taken less than two hours before intense exercise.
• During pregnancy and breastfeeding, up to 200 mg a day is considered safe for the foetus or infant.
• Single doses of 100 mg may affect sleep duration and patterns in some adults, particularly when taken close to bedtime.

These are population figures. Sensitivity differs greatly from one person to another, partly for genetic reasons.

Why the afternoon coffee reaches the night

After a dose, the blood level peaks within about an hour and then halves every few hours. The half-life is usually three to six hours in healthy adults, according to a 2008 review. It is longer in pregnancy and with some medicines, including certain oral contraceptives, and shorter in smokers.

The arithmetic is unforgiving. Two hundred milligrams at 4 pm leaves about 100 mg at 8 pm and 50 mg at midnight for an average metaboliser, and twice that for a slow one. The calculator above shows what remains after any number of hours.

People often say that coffee does not stop them sleeping. They may fall asleep without difficulty and still sleep less deeply. See sleep for executives.

The trap for people who work long hours

Caffeine does not supply energy. It blocks the receptors for adenosine, the compound that builds up during waking hours and produces the pressure to sleep. The tiredness is masked, not removed.

The pattern that results is familiar: a short night, more coffee to get through the day, coffee later in the afternoon, a worse night, and more coffee. Each step is reasonable; the cycle is not.

Breaking it does not require giving up coffee.

1. Set a time for the last cup: eight hours before your usual bedtime is a cautious rule.
2. Keep the total under 400 mg.
3. Delay the first cup by an hour after waking if you tend to slump mid-morning.
4. Replace the afternoon coffee with a ten-minute walk outside, which raises alertness for longer than people expect.

Caffeine and exercise

Caffeine is one of the few supplements with solid evidence of an effect on performance. Doses of 3 to 6 mg per kilogram, taken about an hour before exercise, improve endurance and reduce the perception of effort. The lower end of the range, around 200 mg for a 70-kilogram adult, gives most of the benefit with fewer side effects.

Two cautions for people who train in the evening or in the heat.

• A pre-training dose at 6 pm is still half present at 10 pm.
• Caffeine does not cause dehydration at normal doses, but it raises heart rate and can aggravate palpitations. See hydration.

Signs that you are having too much

• Difficulty falling asleep, or waking at three in the morning.
• Palpitations or a racing pulse at rest. See resting heart rate.
• Anxiety, irritability, tremor.
• Acid reflux.
• Headache on the mornings when you drink less, a sign of withdrawal.

Reducing gradually, by one cup every few days, avoids the withdrawal headache that follows an abrupt stop.

Energy drinks and pre-workout powders

A single can is comparable to a coffee. The risk lies in concentrated products and in combining several sources: a pre-workout powder can contain 300 mg in one scoop. Read the label, and count it in the day's total.

Caffeine powders and tablets make it easy to exceed safe doses by accident and are best avoided.

Who should be more careful

People with heart rhythm disorders, uncontrolled high blood pressure, anxiety disorders or reflux, and pregnant women, have good reason to keep their intake low and to take medical advice. Several medicines interact with caffeine.

The 400 mg figure is a ceiling for healthy adults, not a target. Many people sleep and feel better at half that amount.