Glycine influences sleep through several distinct mechanisms. This process has been studied and is known thanks to the work of Japanese teams, including Yamadera and Kawai.

A drop in body temperature is a natural signal that initiates sleep . Glycine appears to have the ability to “cheat” this mechanism, accelerating peripheral vasodilation (in the hands and feet) and heat release.

Glycine acts on NMDA receptors in the hypothalamus . One study showed that glycine’s hypnotic and hypothermic effects are mediated by NMDA receptors in the suprachiasmatic nucleus. Another study also suggests an effect on orexin neurons (responsible for wakefulness), in which glycine shortened the average duration of wakefulness.

https://pmc.ncbi.nlm.nih.gov/articles/PMC3174993/

A key study conducted by Yamadera in 2007 on volunteers with unsatisfactory sleep: the administration of 3 g of glycine before bedtime was tested for its effect on subjective sleep quality, measured, among others, by the St Mary’s Hospital questionnaire.

Effects noted in studies:

• shortening the time it takes to fall asleep (sleep latency),
• improvement of subjective sleep quality,
• less sleepiness during the day,
• better mental clarity and less fatigue upon waking,
• shortening the sleep onset phase and slow-wave sleep without changing the overall sleep architecture in polysomnographic recordings.

Glycine plays a fundamental role in collagen synthesis.

Collagen has a unique triple-helix structure, composed of three polypeptide chains twisted around each other. This structure requires a repeating motif:

Gly-XY (where X and Y are most often proline and hydroxyproline).

Glycine must be present at exactly every third position in the chain. Glycine is the smallest amino acid, the only one that fits into the tight interior of the triple helix, where the three chains abut closely together. Any other, larger amino acid at this position would disrupt the tight coiling of the helix.

This is why collagen contains as much as about 33% glycine in its composition, much more than is found in the average protein.

The structure of collagen is very strict, even single mutations replacing glycine with another amino acid in the collagen chain can lead to serious connective tissue diseases, e.g. some forms of osteogenesis imperfecta or Ehlers-Danlos syndrome.

Some studies suggest that during intense collagen synthesis (e.g., wound healing, training, aging), the demand for glycine may exceed the body’s biosynthetic capacity; glycine is sometimes referred to as a “conditionally essential” amino acid in such situations.

Studies with glycine supplementation (often combined with vitamin C, a cofactor of proline hydroxylation) have shown the potential to increase markers of collagen synthesis, particularly in the context of tendon and ligament regeneration in athletes.

Glycine has a well-documented role in hepatic metabolism and detoxification.

The liver produces bile acids from cholesterol and then combines them with glycine or taurine before they are secreted into bile, which is crucial for fat digestion, cholesterol regulation and the elimination of fat-soluble substances.

This process is accomplished by a single enzyme: bile acid-CoA:amino acid N-acyltransferase (BAT), which can combine bile acids with both glycine and taurine. In humans, the vast majority of bile acids occur conjugated with glycine (glycine/taurine ratios are usually around 3:1), making glycine the primary “partner” in this process.

Glycine conjugation (Phase II detoxification), independent of bile acids.

The enzyme glycine N-acyltransferase (GLYAT) conjugates glycine with various substances, converting them into forms that are more easily excreted. The glycine conjugation pathway is a two-step enzymatic reaction responsible for the metabolism/detoxification of natural substrates derived from food (e.g., salicylates, polyphenols, medium-chain fatty acids), xenobiotics (e.g., benzoate), and metabolites from organic-acid disorders.

For example, benzoic acid is conjugated with glycine to form hippuric acid and excreted in urine. This is one of the oldest known detoxification pathways in pharmacology.

Glycine is one of the three amino acids that build glutathione, which in the liver:

• neutralizes reactive oxygen species produced during metabolism,
• conjugates toxins in Phase II of detoxification,
• protects hepatocytes against oxidative damage.