Glycine vs Melatonin
Both are commonly discussed for sleep. Glycine and Melatonin both have moderate evidence — Melatonin edges ahead on research volume (20 vs 20 studies referenced).
Evidence last reviewed May 2026
Evidence
🟡Moderate Evidence
🟡Moderate Evidence
Research says
Research agrees
Research agrees
Expert mentions
176
3 recommend
216
3 recommend
Studies
20
referenced
20
referenced
Study dose
3000–15000mg
Sleep studies primarily used 3g (3000mg); the GlyNAC RCT and broader therapeutic reviews reference ranges up to 9–15g daily depending on the health outcome studied. Higher doses have been used in metabolic and aging contexts without notable adverse effects reported.
0mg
The body's endogenous nightly output is approximately 0.1–0.3 mg; studied supplemental doses range from 0.5 mg (sufficient for phase-shifting per expert consensus) up to 10 mg (common OTC doses), though lower doses of 0.5–1 mg are considered more physiologically appropriate for general sleep onset by clinical experts, as higher doses may blunt receptor sensitivity over time.
Best timing
EveningEmpty stomach
EveningEmpty stomach
Who recommends
Caution
Generally safe
Generally safe
Glycine
Key findings
- ·A randomized clinical trial of GlyNAC (glycine + N-acetylcysteine) in older adults found improvements in glutathione deficiency, oxidative stress, mitochondrial dysfunction, inflammation, and physical function, supporting glycine's role as a key substrate for glutathione synthesis.
- ·Glycine is a well-established biochemical precursor to glutathione, creatine, and collagen — three molecules with broad relevance to aging, muscle function, and tissue integrity.
- ·Glycine may be conditionally essential in older adults and under high physiological demand, as endogenous synthesis appears insufficient to meet total-body requirements in these contexts.
Evidence gaps
- ·Most clinical evidence for glycine uses combination products (e.g., GlyNAC, gelatin + vitamin C), making it impossible to determine how much of the observed benefit is attributable to glycine alone versus co-ingredients.
- ·Long-term supplementation trials in diverse human populations are lacking; existing studies are largely short-term and focused on older adults or athletes, leaving gaps for other demographics including those with chronic disease.
Melatonin
Key findings
- ·Melatonin is consistently characterized across multiple reviews as a chronobiotic hormone — a darkness signal produced by the pineal gland — not a sedative or sleep drug.
- ·A meta-analysis on dietary supplements found melatonin associated with improvements in sleep quality outcomes, representing one of the stronger evidence-based applications.
- ·A systematic review and meta-analysis supports melatonin use in ICU patients, suggesting potential benefits in this specific clinical population.
Evidence gaps
- ·Long-term safety and efficacy data for melatonin supplementation in healthy adults and children are largely absent from the reviewed literature, leaving many dosing and duration questions unanswered.
- ·Most reviewed studies are narrative reviews or general overviews rather than large, well-controlled RCTs, making it difficult to establish causality or optimal dosing protocols for specific conditions.