Caffeine is one of the most widely used stimulants in the world. It works by adenosine, a chemical that up in the brain during the and makes us feel sleepy. When adenosine attach to its receptors, we more alert. However, caffeine not remove tiredness; it only it for a . In most adults, about half of the caffeine from a cup of coffee is still in the five hours later, and it can sleep long after the cup is empty.

Most people have noticed that caffeine affects friends and family in very different ways. One person can drink an espresso after dinner and fall asleep easily, while another lies awake after a single cup of tea at lunch. Scientists now know that much of this difference is genetic.

Caffeine is broken down mainly in the liver by an enzyme called CYP1A2. Small differences in the gene that controls this enzyme divide people roughly into fast and slow metabolizers. In fast metabolizers, caffeine leaves the body quickly. In slow metabolizers, it stays in the blood for much longer, so its effects continue into the night.

Other factors also matter. Smoking speeds up the breakdown of caffeine, while pregnancy and some medicines slow it down considerably. Regular coffee drinkers also develop tolerance: their brains respond less strongly to the same amount over time.

For students, the practical advice is simple. Because the effects of caffeine can last many hours, a common recommendation is to avoid it for at least six hours before bedtime. Those who sleep badly after even a little caffeine are probably slow metabolizers and may need an earlier limit.