Individual effects of a nutritional supplementation blend on sleep in professional under-17 footballers: a replicate crossover trial.

To estimate sleep-related mean and individual effects of a supplementation blend comprising 15,000 mg cherry juice concentrate, 220 mg cherry extract, 250 mg cocoa extract, 200 mg of tryptophan, 100 mg of L-5-hydroxytryptophan, 3000 mg of glycine, 300 mg of magnesium, and 200 mg of theanine in youth Middle Eastern football players. Using a two-cycle replicate crossover design, 12 male youth football players (age = 16.1 ± 0.4 years, body mass = 70 ± 8.5 kg, stature = 179 ± 7.5 cm) completed two identical supplementation and two identical placebo (40-g dual-source carbohydrate gel) conditions, each lasting 2 days and in randomized sequences following the rest days subsequent to official match-play. Wrist-worn actigraphy measured time asleep, wake-after-sleep onset, sleep onset latency, and sleep efficiency in free-living conditions. Between-cycle Pearson's correlation coefficients quantified the consistency of the placebo-adjusted sleep responses to active supplementation for the first night and two-night averaged administration effects, respectively. Within-person linear mixed models and random-effects meta-analyses quantified person-by-supplementation response variability. For all administration comparisons and sleep outcomes, the between-cycle correlations were trivial (r range = -0.25 to 0.17, all p ≥ 0.43), with linear mixed modelling and meta-analytic approaches indicating trivial replicate-averaged supplementation effects. Participant-by-supplementation variability in sleep responses was trivial, indicated by the individual treatment estimates showing strong shrinkage toward the mean. Aggregated sets of n-of-1 trials revealed trivial replicate-averaged treatment effects and a lack of meaningful individual differences in placebo-adjusted sleep responses to blend supplementation in male youth Middle Eastern football players.