For some people, drinking an afternoon cup of coffee is enough to disrupt their night’s sleep, but others can drink coffee or an energy drink at night and shortly after fall asleep without a problem. A reality that shows caffeine does not affect everyone the same. Experts say genetics, the rate of caffeine metabolism in the liver, smoking and certain medications, pregnancy and the level of habituation all play a role in this difference, and they recommend that sensitive individuals avoid consuming caffeine at least six hours before bedtime.
According to the BBC World Service, caffeine is by far the most commonly used stimulant of the brain and nervous system in the world. More than two billion cups of coffee are drunk worldwide every day. This amount of caffeine does not include energy drinks, Coca-Cola and some chocolates, gums and sports supplements, which also contain this chemical.
Caffeine can make us more alert, improve concentration and delay fatigue, but excessive consumption may cause headache, restlessness and tremor, palpitations and restless nights. Experts emphasize that caffeine does not provide more energy to the body, but helps hide the feeling of fatigue. It produces this effect by blocking the action of adenosine; a chemical that accumulates in the brain during the day and makes us feel sleepy.
Why does caffeine affect people differently?
One main factor is how long caffeine stays in the body. After drinking a cup of coffee, caffeine enters the bloodstream and quickly reaches the brain. At the same time, an enzyme in the liver breaks down the caffeine and then it is excreted from the body through the urine.
According to Dr Marilyn Cornelis, Associate Professor of Nutritional Sciences at Northwestern University in Chicago, this clearance process can take between six and twenty hours depending on the individual. She says: "A person may drink their first cup of coffee and not feel any particular effect, but two or three hours later the caffeine may still remain in the body and if a second cup is added, that amount may be enough to upset the usual balance and the person may feel a little anxious."
The speed of processing caffeine in the liver depends on a specific gene called CYP1A2. Interestingly, a person's genetic ancestry may also partly influence this process. A study published this year showed that the bodies of Europeans and South Americans on average break down caffeine faster.
However genetic makeup is by no means the whole story. It appears that smoking - unlike using electronic cigarettes - stimulates the liver to break down caffeine much faster and can increase the rate of this process by up to 65 percent.
This may explain why smokers usually drink more coffee and why those who quit smoking may suddenly feel that their usual cup of coffee affects them much more strongly than before.
Some antibiotics, birth control pills and other prescription medications can have the opposite effect. Such that caffeine remains in the body longer and its effects last longer. This situation also occurs during pregnancy, when hormonal changes reduce the activity of the liver enzyme that breaks down caffeine.
Another factor is the level of tolerance. People who drink coffee regularly gradually become less sensitive to caffeine, because the body adapts to the presence of this chemical.
What do experts recommend?
Sleep researchers often recommend that people avoid consuming caffeine at least six hours before bedtime. Dr Lindsay Browning, psychologist and sleep specialist, says evidence shows that caffeine may not only prevent falling asleep, but also affect sleep quality. She says: "I usually advise people who go to bed around 10pm not to consume caffeine after about 2pm. But setting a universal, definitive and inflexible rule is not possible, because the complexities of the issue and individual differences are very important."
The UK National Health Service advises pregnant women to limit their daily caffeine intake to less than 200 milligrams, equivalent to about two large cups of filtered coffee, because studies have linked caffeine consumption with low birth weight and some other complications.
For other people, the main message is moderation, because sensitivity to caffeine varies greatly from person to person. Peter Rogers, Emeritus Professor of Nutritional Sciences at the University of Bristol, says: "Caffeine is relatively safe, but if you consume a very large amount of it, you will experience unpleasant shaking and restlessness."
Dangerous excessive caffeine consumption is very rare and severe poisoning cases are almost always reported with products containing high levels of caffeine, that is products that may contain several grams of caffeine swallowed within minutes.
The good news for coffee lovers is that the habitual morning habit of drinking coffee is associated with better health outcomes. Although researchers cannot prove that coffee alone is the cause of these benefits, decades of research show that people who drink coffee in moderation are generally less likely to develop several major diseases and may even live slightly longer than those who do not drink coffee.