Glycine

Glycine is the simplest amino acid and the most abundant in the human body. While the body synthesizes it endogenously, production falls short of total requirements , making supplementation particularly relevant for sleep, connective tissue maintenance, and metabolic health.

Sleep Quality {#sleep-quality}

The most well-studied use of glycine supplementation is pre-sleep dosing for improved sleep quality. A randomized, double-blind, placebo-controlled trial (PMC3328957) found that 3 g of glycine taken before bed significantly improved subjective sleep quality, reduced sleep latency, and enhanced next-day alertness and performance in partially sleep-restricted volunteers. Participants reported less fatigue and better cognitive function the following morning despite the same total sleep time.

Core Body Temperature Pathway

The mechanism is unusually well-characterized for a sleep supplement. Glycine acts on NMDA receptors in the suprachiasmatic nucleus (SCN) , the brain’s central circadian clock , to trigger peripheral vasodilation. Increased blood flow to the skin dissipates body heat, lowering core body temperature. This temperature drop is one of the primary physiological signals that initiates sleep onset. A separate animal study (PMC4397399) confirmed this hypothermic pathway using NMDA receptor antagonists to block the effect, directly linking glycine’s sleep-promoting action to SCN signaling rather than sedation.

Polysomnographic recordings in human trials show increases in slow-wave sleep and reductions in wakefulness during the night, consistent with glycine shifting the body more rapidly into restorative sleep stages rather than simply inducing grogginess.

Collagen Synthesis {#collagen-synthesis}

Glycine makes up roughly one-third of all amino acids in collagen by mass, and is the most limiting factor in collagen synthesis. Endogenous production provides approximately 3 g/day, while full collagen turnover requires an estimated 10–15 g/day , a gap that cannot be closed by diet alone without deliberate supplementation.

Supplemental glycine supports the continuous production of collagen in joints, skin, tendons, ligaments, and bone matrix. This is relevant for recovery from connective tissue injury, age-related collagen decline, and athletes placing high mechanical loads on tendons and cartilage. A 2024 review (PMC11510825) highlighted glycine’s role in supporting physical recovery and connective tissue integrity, noting that it is undersupplied in typical Western diets , particularly in those who do not consume collagen-rich foods such as bone broth or skin-on cuts.

Metabolic Health {#metabolic-health}

Emerging evidence suggests glycine may improve markers of insulin sensitivity and metabolic syndrome. Glycine is required for bile acid conjugation and plays a role in glutathione synthesis, both of which influence metabolic function. Observational data shows lower fasting plasma glycine in individuals with metabolic syndrome and type 2 diabetes. Intervention studies are smaller and less consistent than the sleep evidence, but the mechanistic basis is plausible.

Dosing

For sleep, 3 g taken 30–60 minutes before bed is the dose used in the primary RCTs and the most commonly recommended starting point. For collagen support, higher doses of 5–15 g/day are used, often combined with vitamin C to support hydroxylation steps in collagen synthesis.

Glycine is very well tolerated. No established upper limit exists. Research using doses up to 60 g/day for metabolic and psychiatric conditions has not identified significant adverse effects. Mild gastrointestinal sensitivity is occasionally reported at high doses.