Glycine vs L-Theanine
Both are commonly discussed for sleep. Glycine and L-Theanine both have moderate evidence — L-Theanine edges ahead on research volume (20 vs 20 studies referenced).
Evidence last reviewed May 2026
Glycine
Amino Acid
L-Theanine
Amino Acid
Evidence
🟡Moderate Evidence
🟡Moderate Evidence
Research says
Research agrees
Research agrees
Expert mentions
176
3 recommend
191
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.
100–400mg
Studies across anxiety, sleep, and cognitive outcomes have generally used doses in the 100–400 mg range, with many individual experts noting personal use around 100–200 mg; optimal dose may vary by goal and individual sensitivity.
Best timing
EveningEmpty stomach
MorningEvening
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.
L-Theanine
Key findings
- ·Multiple systematic reviews suggest L-theanine may modestly reduce subjective stress and anxiety, but effect sizes are generally small and study quality is variable.
- ·A systematic review and meta-analysis on sleep found some supportive evidence for L-theanine improving sleep outcomes, though results were not uniformly strong.
- ·The L-theanine and caffeine combination appears to be the best-supported application, with reviews noting potential reduction in caffeine-induced jitteriness and possible cognitive benefits.
Evidence gaps
- ·There is a lack of large, well-powered, long-term RCTs in general adult populations, meaning chronic effects on anxiety, sleep, and cognition remain poorly characterized.
- ·Optimal dosing, timing, and formulation of L-theanine supplementation have not been systematically established across studies, limiting practical guidance.