How does caffeine act on the brain?

Caffeine is an alkaloid found in tea, coffee, and cocoa. It acts as an antagonist of adenosine receptors: by lodging itself on these receptors, it prevents adenosine from signaling fatigue, which pushes back the feeling of drowsiness and increases alertness. It is a stimulant of the central nervous system, not a real source of energy.

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Caffeine in Dr Stone S1E13

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Before a decisive showdown, Senku brews a beverage meant to power up his allies. He counts on caffeine extracted from tea leaves, combined with honey for the sugar boost and with sweet flag, a plant with a soothing reputation, to compose a stimulant drink built for battle.

The subject in depth

Caffeine is an alkaloid of the methylxanthine family, naturally present in tea leaves, coffee beans, cocoa beans, and a few other plants ^[Wikipedia]. It is the most widely consumed psychoactive substance in the world. In the plant, it plays the role of a natural pesticide, deterring or paralyzing the insects that attack the leaves.

Its mechanism of action is well understood. Over the course of a day, a molecule called adenosine builds up in the brain and binds to its receptors, which slows neuronal activity and gradually settles in the feeling of fatigue. Caffeine’s molecular shape is close enough to adenosine’s for it to lodge itself on the same receptors without activating them: it acts as an antagonist. By occupying the spot, it prevents adenosine from transmitting its drowsiness signal ^[Wikipedia]. The result is not an intake of energy, but a temporary release of the fatigue brake, which translates into heightened alertness, reduced drowsiness, and slight cardiac and respiratory stimulation.

Diagram of caffeine blocking adenosine receptors in the brain, the mechanism behind its stimulant effect on alertness — Caffeine squats on the receptor and blocks the fatigue signal

Tea leaves are a particularly convenient source in a survival context. The tea plant, Camellia sinensis, concentrates caffeine in its young leaves, at levels comparable to or higher than those of coffee beans at equal mass ^[Wikipedia]. A simple hot-water infusion is enough to extract the alkaloid, which is water-soluble. The added honey provides quickly assimilated sugars, a very real source of metabolic energy, while sweet flag stems from a traditional use as a soothing plant whose effects remain poorly established scientifically.

The fact-checker's eye

Senku’s brew is presented as a near-decisive booster, capable of transforming the fighters’ condition. The pharmacological reality is more measured. Caffeine improves alertness, delays the feeling of fatigue, and can slightly increase endurance and perceived effort, but it creates no additional strength or energy: it temporarily masks fatigue without removing it, and the crash comes afterward. The real gains in physical performance are modest and vary from one person to another, and part of the perceived effect comes down to expectation and suggestion, in other words, a placebo effect. As for the sweet flag added to the mix, its traditional soothing reputation does not rest on solid evidence. In short, the chemistry of caffeine is accurate, but the anime clearly exaggerates its effect by turning it into a genuine power-up for combat.

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Published on August 8, 2026

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Caffeine in Dr Stone S1E13

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