Our pick · Glycine

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Glycine

The simplest amino acid with surprisingly good RCT evidence for improving sleep quality and supporting collagen synthesis.

By editorialUpdated 2026-05-251 min read

What it's actually good for

Glycine is the smallest and simplest amino acid, yet it plays an outsized role in several physiological processes. The most compelling supplementation evidence is for sleep: multiple RCTs demonstrate that 3 g of glycine taken before bedtime improves subjective sleep quality, reduces the time it takes to fall asleep, and decreases next-day drowsiness and fatigue. The mechanism appears to involve glycine lowering core body temperature via peripheral vasodilation and modulating NMDA receptors in the suprachiasmatic nucleus. Importantly, glycine does not cause morning grogginess or alter sleep architecture the way sedative sleep aids do. Separately, glycine is a major building block of collagen (making up about 33% of collagen's amino acid content), and supplementation may support collagen turnover in skin, joints, and connective tissue, though direct clinical outcome data for joint health from glycine alone is still limited. It earns a B rather than an A because the sleep trials, while consistent, are small, and the collagen-related benefits are more mechanistically inferred than clinically proven at this point.

Claim-by-claim

Each claim graded independently

The overall grade is the floor. Some claims are stronger or weaker than the headline.

B

Improves subjective sleep quality and next-day alertness

Several small but well-designed RCTs show that 3 g of glycine taken before bed reduces sleep onset latency, improves subjective sleep quality, and decreases next-day fatigue and sleepiness without altering sleep architecture on PSG.

B

Supports collagen synthesis and joint health

Glycine is a primary component of collagen (roughly one-third of its amino acids). Supplementation increases serum glycine and provides substrate for collagen production, though direct joint outcome data from RCTs is limited.

Sources

3 cited
[01]RCTGlycine ingestion improves subjective sleep quality in human volunteers, correlating with polysomnographic changesInagawa K, Hiraoka T, Kohda T, Yamadera W, Takahashi M. Sleep Biol Rhythms. 2006
[02]RCTThe Sleep-Promoting and Hypothermic Effects of Glycine are Mediated by NMDA Receptors in the Suprachiasmatic NucleusKawai N, Sakai N, Okuro M, et al.. Neuropsychopharmacology. 2015
[03]MECHNew therapeutic strategy for amino acid medicine: glycine improves the quality of sleepBannai M, Kawai N. J Pharmacol Sci. 2012

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