Effects of Three Consecutive Days of Diurnal Intermittent Fasting on Cognitive Function, Psychomotor Vigilance, and Sleep Quality in Adolescent Male Soccer Players: A Quasi-Experimental Parallel-Group Study

Aim: While voluntary Islamic intermittent fasting (IF) of three consecutive days (mimicking practices such as Ayyam al-Bid) may be observed by Muslim athletes, its short-term effects on cognitive function and sleep quality in adolescent team-sport athletes remain unexamined. This study investigated the impact of three successive days of dawn-to-sunset IF on memory, attention, psychomotor vigilance, and subjective sleep quality in young male soccer players. Methods: A quasi-experimental, parallel-group design was employed. Thirty-five male soccer players (age: 15.6 ± 0.8 years; body mass: 55.07 ± 6.53 kg) were allocated to an IF group (n = 17) or a non-fasting control group (n = 18). The fasting protocol comprised approximately 14 consecutive hours of complete food and fluid abstinence per day (from 04:40 to 18:36, with daily variations of −3 min for the dawn and +2 min for the sunset, local time). Cognitive function was assessed using the CogniFit Cognitive Assessment Battery (CAB) and the Psychomotor Vigilance Test (PVT), and subjective well-being was quantified via the Hooper Index. Both assessment sessions were conducted between 14:00 and 17:00 to control for diurnal variation. Results: The Group × Time interaction for single-item subjective sleep quality reached nominal significance at the unadjusted threshold (Z = 2.18, p = 0.029, r = 0.37) but did not survive Bonferroni correction for the four Hooper Index comparisons (α = 0.0125); this result is therefore interpreted as hypothesis-generating only, reflecting a within-group deterioration in the control group (p < 0.05) alongside a non-significant marginal improvement in the fasting group. No statistically significant Group × Time interaction was detected for any memory subdomain (all p ≥ 0.419, r ≤ 0.14), attention component (all p > 0.176, r ≤ 0.23), or PVT parameter (all p > 0.192, η2p/r ≤ 0.22) under the present study conditions. Within-group improvements in memory and attention observed in both groups were consistent with test–retest learning effects. Conclusions: No statistically significant Group × Time interaction was detected for memory, attention-related executive functions, or psychomotor vigilance in young male soccer players under the present study conditions; the study was not designed as an equivalence or non-inferiority trial and cannot exclude effects smaller than the targeted medium magnitude, alongside a nominally divergent but statistically uncorrected sleep quality trajectory relative to non-fasting controls, which should be interpreted as hypothesis-generating pending replication. These findings provide preliminary, hypothesis-generating evidence that no statistically significant cognitive or psychomotor impairment was detected under the present study conditions; because the study was not designed as an equivalence or non-inferiority trial, compatibility between short-term voluntary IF and adolescent athletic training cannot be inferred from the absence of a significant interaction alone.