Caffeine has recently acquired something of a bad reputation. Concerns range from highly caffeinated energy drinks disrupting children’s concentration in class, to excessive teas and coffees leaving adults unable to sleep at night. For many, it has become a substance to avoid.
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Yet caffeine is not without its merits. It is, after all, a psychoactive substance – the most widely consumed drug in the world, in fact – and chances are you are under its influence as you read this. Like many drugs, the effect depends on the dosage. Taken in moderation, caffeine has historically provided the clarity and vigour that helped fuel Europe’s Enlightenment and even supported the shift from agricultural to industrial work.
The key lies in how much, and when, it is consumed. Traditional tea and coffee have been staples for centuries, but today’s energy drinks and caffeine tablets deliver much higher concentrations. This surge has prompted growing scientific research into caffeine’s effects, as researchers seek to understand not only its impact, but also why individuals respond so differently.
Evidence now suggests caffeine may do more than simply keep us alert. Regular, moderate consumption of coffee or tea appears linked to reduced risks of certain illnesses, including diabetes and some cancers. This raises the question: does caffeine really deserve its negative image?
Both the European Food Safety Authority and the US Food and Drug Administration recommend a daily intake of up to 400mg – roughly two to three mugs of filter coffee – as safe for healthy adults. However, the timing of consumption also matters.
Researchers in Australia and the UK, writing in Sleep Medicine Reviews, recommend finishing your last tea or coffee 8 hours and 48 minutes before bedtime. Those using pre-workout caffeine supplements, which often contain double the caffeine of a standard cup, should stop at least 13 hours and 12 minutes before going to bed.
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But the challenge with offering such precise advice is that caffeine affects people differently. Sensitivity varies, and so does the length of time it lingers in the body. In adults, its half-life – the period required for the substance to lose half its strength – ranges from three to seven hours. This variation is largely genetic.
During the day, the brain accumulates a chemical called adenosine, which attaches to receptors on nerve cells, slowing them down and making us feel drowsy. Caffeine competes with adenosine by binding to the same receptors, keeping neurons active and helping us stay alert. It also stimulates the pituitary gland, prompting the adrenal glands to release adrenaline. This in turn quickens the heartbeat and raises blood pressure.
Over time, however, the brain adapts. As Professor Jennifer Temple from the University at Buffalo explains, regular caffeine intake encourages the brain to create additional receptors for adenosine, and the body produces more of it to compensate. This is why larger doses are needed to achieve the same effect. Such adaptations can take place within just a week.
Beyond adaptation, genetic differences play a role. Caffeine is mainly metabolised in the liver by an enzyme called CYP1A2, and the gene that produces this enzyme varies significantly from person to person. This helps explain why some can enjoy an evening espresso without consequence, while others are left wide awake into the early hours.
