Caffeine blocks signals that make the body feel sleepy, which is why it is so widely used.

The longer version

Adenosine accumulates in the brain during waking hours and binds to receptors that promote drowsiness. Caffeine resembles adenosine closely enough to occupy those receptors without activating them, so the signal is blocked rather than removed. Receptors elsewhere in the body respond as well, which is part of why large doses feel unpleasant.

The pressure keeps building underneath, which is why drowsiness can return once the compound clears. Sensitivity varies with genetics, habitual intake and liver enzymes, so the same dose affects people differently. Caffeine is the most widely consumed psychoactive substance, occurring naturally in coffee, tea, cacao and many other plants as an insect deterrent.

Where this entry sits

This collection gathers curious details about sleeping and dreaming, from nightly brain stages and body clocks to naps, yawns and unusual animal habits. It also covers how bedtime customs have changed across centuries, without offering any advice. The full list sits on the Sleep and Dreams page, where every entry is listed in order.