The effect of probiotic and synbiotic supplementation on sleep parameters in exercised population: a systematic review and synthesis without meta-analysis (SWiM) of randomized controlled trials

BACKGROUND: Sleep is crucial for recovery and optimal performance in athletes; however, poor sleep is common during periods of intensive training or competition. The microbiota-gut-brain axis suggests probiotics and synbiotics could modulate sleep, but evidence in exercised populations is limited. OBJECTIVE: To systematically review randomized controlled trials (RCTs) assessing the effects of probiotic and synbiotic supplementation on sleep in exercised populations, using a Synthesis Without Meta-analysis (SWiM) approach. METHODS: PubMed, Scopus, Web of Science, and ProQuest were searched up to September 2025. Eligible studies were RCTs including exercised individuals, testing probiotics or synbiotics, and reporting sleep outcomes. Data extraction, risk of bias assessment (RoB 2), and narrative synthesis followed SWiM guidelines. Benefits were most consistent for subjective sleep quality and, in some cases, sleep latency. Secondary outcomes showed occasional reductions in stress, anxiety, and fatigue. CONCLUSION: Probiotic and synbiotic supplementation may improve sleep in exercised populations, especially perceived quality and latency. Evidence supports cautious, adjunctive use, but larger, standardized trials are needed to confirm effects.PROSPERO registration number: CRD420251151264.