Differential Time-of-Day Effects of Caffeine Capsule and Mouth Rinse on Cognitive Performance in Adolescent Male Volleyball Athletes: A Randomized Crossover Investigation

Caffeine is widely used to enhance cognitive performance, but its efficacy may vary with the administration route and circadian timing. This study compared the acute effects of caffeine capsule ingestion and caffeine mouth rinsing on cognitive performance across morning, midday, and evening sessions in well-trained, adolescent male volleyball players. Twenty-four athletes completed three randomized, double-blind, crossover trials involving a caffeine capsule (3 mg·kg^-1), a caffeine mouth rinse of the same dose (expectorated), and a placebo. Cognitive performance was assessed using simple and choice reaction time tests and the Stroop task, alongside a side-effects questionnaire. Both caffeine forms improved performance versus the placebo, with the greatest enhancements occurring at midday and moderate benefits evident in the morning. Capsule ingestion produced the most consistent improvements across reaction speed and executive control, whereas mouth rinsing elicited smaller, task-dependent effects, particularly at midday. No consistent or practically relevant benefits were observed for either caffeine condition in the evening, when cognitive performance was naturally highest. Side effects were mild and infrequent, with occasional headaches after capsule ingestion. These findings indicate that caffeine capsules most effectively enhance cognitive performance when baseline alertness is suboptimal, while caffeine mouth rinsing represents a practical ingestion-free alternative with moderate efficacy.