Caffeine effects can vary dramatically from one person to another. Some people can drink coffee in the evening and fall asleep soon afterward, while others may spend hours awake after a single cup.
Emily, a 20-year-old coffee drinker in London, says caffeine has little effect on her.
She usually drinks two cups of coffee each day. She may also have an energy drink and several cans of cola without feeling particularly alert or restless.
For other people, even one late afternoon coffee can interfere with sleep.
So why does caffeine affect people so differently?
How Caffeine Works
Caffeine ranks among the world’s most widely used psychoactive substances. People consume it through coffee, tea, cola, energy drinks, chocolate and some supplements.
The chemical can increase alertness and improve concentration. However, large amounts can also cause headaches, shakiness, a racing heartbeat and sleep problems.
Caffeine does not provide the body with extra energy. Instead, it temporarily masks tiredness.
It works mainly by blocking adenosine, a chemical that gradually builds up in the brain during the day. Adenosine makes people feel sleepy, so blocking its action can make people feel more awake.
How Long Does Caffeine Stay in Your Body?
One major difference involves the speed at which the body removes caffeine.
After someone drinks coffee, caffeine enters the bloodstream and reaches the brain quickly. The liver then breaks down the chemical before the body removes it through urine.
Dr Marilyn Cornelis, an associate professor of nutrition at Northwestern University, says this process can take roughly six to 20 hours, depending on the person.
That difference can change how strongly someone reacts to a cup of coffee.
For example, a person may drink coffee without noticing an immediate effect. However, caffeine can remain in the body for several hours. A second cup can then add to the amount already circulating in the system.
Your Genes Can Affect Caffeine Sensitivity
Genetics also play an important role.
A gene called CYP1A2 helps control how quickly the liver processes caffeine. Differences in this gene can therefore influence how long caffeine remains in the body.
Research has also found differences in caffeine metabolism between populations. However, genetics alone cannot explain why two people can have very different reactions to the same drink.
Other lifestyle and health factors also matter.
Smoking Can Change Caffeine Metabolism
Smoking can speed up the body’s caffeine breakdown. Research suggests cigarette smoking can increase caffeine clearance substantially.
That may help explain why some smokers consume more coffee.
The effect can also change after someone quits smoking. Their normal cup of coffee may suddenly feel much stronger because their body no longer processes caffeine as quickly.
Vaping does not appear to have the same established effect on caffeine metabolism.
Medicines and Pregnancy Can Slow the Process
Some medicines can make caffeine remain in the body for longer.
Certain antibiotics, contraceptive medicines and other prescription drugs can affect the liver enzyme responsible for caffeine metabolism.
Pregnancy can also change the way the body handles caffeine. Hormonal changes can slow caffeine breakdown and prolong its effects.
Tolerance provides another important factor.
Regular coffee drinkers often become less sensitive to caffeine because the body adapts to repeated exposure. However, that tolerance can fall after someone stops consuming caffeine for several weeks.
As a result, the first coffee after a long break may produce a much stronger reaction.
How Caffeine Can Affect Sleep
Sleep experts often advise people to avoid caffeine for several hours before bedtime.
Dr Lindsay Browning, a psychologist and sleep specialist, says caffeine can affect both the ability to fall asleep and sleep quality.
For someone who goes to bed around 10pm, she suggests avoiding caffeine from about 2pm.
However, individual sensitivity makes a single rule difficult to apply to everyone.
Someone who quickly processes caffeine may tolerate an afternoon coffee. Another person may still feel its effects late at night.
How Much Caffeine Is Safe?
The NHS advises pregnant women to limit caffeine consumption to below 200mg per day.
For most other adults, moderation remains the key message.
Professor Peter Rogers, emeritus professor of nutrition at the University of Bristol, says caffeine is generally safe when people consume reasonable amounts.
However, very high doses can cause unpleasant symptoms such as severe jitters and a rapid heartbeat.
Severe caffeine poisoning remains rare. Serious cases usually involve concentrated caffeine products rather than ordinary coffee.
Is Coffee Good for You?
Coffee drinkers also have some encouraging research to consider.
Decades of studies have linked moderate coffee consumption with better health outcomes and lower risks of several major diseases.
However, researchers cannot say that coffee alone causes these benefits. Other lifestyle factors may also influence the results.
The important point remains individual sensitivity.
Some people can enjoy coffee late in the day without major problems. Others need to stop much earlier.
Understanding how your body responds can help you balance caffeine’s benefits with its possible effects on sleep and wellbeing.
