“Coffee just doesn’t have much effect on me,” says 20-year-old Emily, who is leaving a cafe in central London at 18:00 BST.

“I’ll normally have two cups a day and then maybe an energy drink too. And then I can drink coke all night and feel completely fine.”

We’ve all got a friend like Emily. The one who can knock back a double espresso or even an energy drink in the evening and still drift off to sleep an hour or so later.

For many people, though, the effects of caffeine can linger for far longer.

Why then can some people drink coffee before bed and sleep like a baby, while others lie awake staring at the ceiling?

Caffeine is by far the most used psychoactive substance in the world.

More than two billion cups of coffee are drunk each day and that’s before we start talking about cola, energy drinks, plus some chocolate, chewing gum and sports supplements, which also contain the chemical.

It can make us feel more alert, improve concentration and stave off tiredness. Too much can cause headaches, jitters, a racing heart and restless nights.

Caffeine doesn’t give your body extra energy, it helps mask tiredness, experts say.

It does this by blocking the action of adenosine, a chemical that builds up in the brain throughout the day and makes us feel sleepy.

One key factor is how long caffeine stays in the body.

After drinking a cup of coffee, it is absorbed into the bloodstream, where it quickly reaches the brain.

All this time it’s also being broken down – or metabolised – by an enzyme in the liver and then flushed out in our urine.

That clean-up process can take between six and 20 hours depending on the individual, says Dr Marilyn Cornelis, an associate professor of nutrition at Northwestern University in Chicago.

“It’s possible to have a first cup of coffee and maybe not feel the effects,” she says.

“But then two or three hours later, the caffeine is still lingering in the system, and, if you add a second cup, that might be enough to tip the scales and you start feeling a little anxious.”

How quickly the liver can processes the caffeine depends on a single gene – known as CYP1A2.

Intriguingly it might be influenced, at least to some extent, by your ancestry.

A study published this year, external found that Europeans and South Americans tend to break down caffeine more quickly on average.

Your DNA is far from the whole story. Smoking, but probably not vaping, appears to stimulate the liver to break down caffeine much more quickly, speeding up the process by as much as 65%.

That may help explain why smokers tend to drink more coffee and why people who quit can suddenly find their usual cup feels much stronger than before.

Some antibiotics, contraceptive pills and other prescription medicines can have the opposite effect, allowing caffeine to stay trapped in the body for longer and prolonging the effects.

That’s also the case during pregnancy, when hormonal changes suppress the liver enzyme that breaks down caffeine.

Then there is the question of tolerance.

Regular coffee drinkers become less sensitive to caffeine as the body adapts to the presence of the chemical.

If you then go without the drink for a few weeks, those adaptions in the brain begin to reverse, meaning the first coffee back might leave you feeling unusually wired or jittery.

Sleep researchers often recommend avoiding caffeine for at least six hours before bedtime.

Dr Lindsay Browning, a psychologist and sleep expert, says the evidence suggests caffeine might not just stop you dozing off but can affect the quality of sleep as well.

“I’ll suggest to people that they might want to avoid caffeine after about 2pm if they are going to bed around 10pm,” she says.

“But a blanket hard-and-fast rule isn’t helpful, because it doesn’t really allow for the intricacies and the individualisation of what’s really going on.”

The NHS advises pregnant women, external to keep caffeine intake below 200mg a day, about two mugs of filter coffee, as studies have linked it to low birth weight and other complications.

For other adults the key message is moderation, as sensitivity to caffeine varies so much from person to person.

“Caffeine is relatively safe,” says Peter Rogers, emeritus professor of nutrition at the University of Bristol.

“If you consume a great deal of it, you just start feeling jittery and unpleasant and you start realising, ‘I need to stop this, there’s no point taking any more’.”

Caffeine overdoses are very rare and severe poisonings almost always involve concentrated caffeine products, where several grams can be swallowed in a matter of minutes.

For coffee lovers the good news is that, if anything, that regular (if expensive) morning habit is linked to better health outcomes.

While researchers cannot prove that coffee is the only reason, decades of studies suggest that people who drink it in moderation tend to have lower risks of several major diseases and may even live slightly longer than non-drinkers.