Glycine Bioavailability and Sleep-Related Outcomes Following Oral Glycine and Collagen Supplementation-A Scoping Review.

Glycine is a conditionally essential amino acid with plausible roles in sleep regulation through thermoregulatory and glycinergic/N-methyl-D-aspartate signaling pathways. Collagen is rich in glycine and thus, collagen-based supplements may represent a practical dietary source of glycine and could plausibly influence sleep-related outcomes. We conducted a scoping review to map the clinical literature on glycine bioavailability from dietary supplements and to evaluate whether available evidence supports glycine as a biologically plausible candidate mechanism underlying sleep-related effects of glycine- and collagen-containing supplements. Following Joanna Briggs Institute guidance and PRISMA-ScR reporting standards, we searched PubMed, Embase, Web of Science, and the Cochrane Central Register of Controlled Trials from inception through July 9, 2026. A total of 21 records comprising 22 unique trials met the inclusion criteria. Of these, 14 trials examined pharmacokinetic glycine availability from glycine-, collagen-, or other glycine-containing supplements, 3 investigated supplemental glycine and sleep-related outcomes, and 5 studies reported across 4 records evaluated collagen or collagen peptides and sleep-related, fatigue, or recovery-related outcomes. Across pharmacokinetic trials, glycine-containing supplement interventions generally increased circulating glycine concentrations, with some evidence of dose dependence. All 3 glycine sleep trials reported benefits on at least 1 sleep-related outcome. Among collagen peptide trials, 1 placebo-controlled trial reported fewer polysomnographic awakenings, 1 placebo-controlled trial reported lower fatigue scores, and active-comparator studies with alpha-lactalbumin did not show superiority of collagen peptides for sleep-related outcomes. Overall, current evidence supports biological plausibility for glycine as a candidate mechanism that may contribute to collagen-related sleep effects, but mediation has not been directly tested. Larger mechanistic trials with standardized sleep outcomes are needed.