A study asked whether drinking sparkling water during a three-hour esports session keeps a player sharper than plain water. Fourteen casual gamers played eFootball twice, once with each drink, while researchers tracked their pupils, their reactions, and how tired they said they felt.
The caveats first, because they matter. The study was funded by Asahi Soft Drinks, and two of its four authors, Kosugi and Mizuno, work there. It has fourteen participants, thirteen of them men. The players knew which drink they were getting, because you always know when water is fizzy. This is a pilot, not settled science.
What survives those caveats is the pupil. In the plain-water session, after about two hours the players' pupils constricted and their reactions on a flanker task slowed. The sparkling-water session held steady. Subjective tiredness climbed only in the plain-water group, and only after the pupil had already begun to constrict.
Pupil size tracks the brain's arousal system, the same norepinephrine pathways behind attention and executive control. The ordering is the point: the pupil narrowed before the players felt tired. There is a window, somewhere past the two-hour mark, where you are measurably slower and more distractible and still feel fine.
Watch the pupil
Watch the eye through a three-hour session, then toggle to sparkling water and replay. In still water the pupil constricts first while the "feeling" gauge stays bright — objective decline before subjective fatigue. That lag is the blind window. The pupil sizes are illustrative, scaled to the study's effect sizes; the study measured hourly, so the exact onset is interpolated.
The mechanism is a hypothesis, and the authors are clear about it. Carbonation irritates TRP channels in the throat, which feed the trigeminal nerve, which projects to the locus coeruleus, the brainstem's arousal hub. They did not test that chain directly. It would explain why a drink with no calories and no caffeine did anything at all, but it stays unproven.
There is also a result I do not trust. Sparkling water was linked to fewer fouls in the virtual match at hour one, with no change in shots, passes, or goals. It is a nice, game-native detail, but at fourteen people and with the blinding impossible, it is a curiosity. The pupil result is the one that holds.
Editorial
The honest sensor
What interests me here is not the drink. The body has a fatigue sensor you cannot consciously read, and it reports before you feel anything. The pupil is a tachometer for a resource you will not notice draining. A long coding session happens inside that blind window.
I do not expect to change what I drink because of a fourteen-person Asahi study. I do expect to keep an eye on the eye. A cheap, continuous, objective signal that says you are degrading before you feel it is worth more than any energy drink.