Also known as: aminoacetic acid, glycin
Three grams before bed, tastes faintly sweet, costs almost nothing, and has a plausible mechanism for better sleep quality — on a thin trial base.
A small set of Japanese trials shows improved subjective sleep quality and reduced next-day fatigue, with a well-characterised thermoregulatory mechanism. The literature is limited in size and largely from one research group.
Glycine is the simplest amino acid there is, your body makes plenty of it, and it is one of the cheapest things in the supplement aisle. It also has a mechanism for sleep that is unusually well characterised for a compound this obscure.
Falling asleep requires your core temperature to drop, and the body accomplishes that by dilating blood vessels in the hands and feet to radiate heat. Glycine appears to accelerate that process by acting on NMDA receptors in the suprachiasmatic nucleus. Rodent work traced the pathway specifically: the sleep effect disappears when those receptors are blocked, and it comes with the expected measurable fall in core temperature. That is a considerably more concrete story than most sleep supplements can tell.
The human trials are where enthusiasm should moderate. A small series, mostly from Japanese researchers with commercial connections to glycine production, found that 3 g before bed improved subjective sleep quality in people with poor sleep, reduced next-day fatigue, and — in one study using polysomnography — shortened the time to fall asleep and to reach slow-wave sleep. Participants reported feeling more clear-headed the following day. The results are consistent in direction but the total number of people studied is modest, and independent replication is thin.
What makes glycine worth including anyway is the risk-reward. It is a dietary amino acid, safe at doses far above 3 g, non-sedating, non-habit-forming, and costs a few cents a night. There is no morning hangover, unlike higher-dose melatonin, and no dependency question, unlike sleep medication. If it does nothing for you, you have lost very little.
A note on collagen, since it comes up: collagen peptides are about a third glycine, so 10 g of collagen delivers roughly 3 g — which may explain a portion of the sleep benefit some people report from it. Glycine also pairs sensibly with magnesium glycinate, where the glycine is already half the molecule. Buy it as bulk powder; it dissolves easily and tastes mildly sweet.
Glycine acts on NMDA receptors in the suprachiasmatic nucleus, the brain's master clock, triggering peripheral vasodilation. That dumps heat through the hands and feet and lowers core body temperature — the same physiological drop that normally initiates sleep. It is also an inhibitory neurotransmitter in its own right at glycine receptors.
Very safe. Glycine is a normal dietary amino acid consumed in grams daily from protein, and trials have used far higher doses without issue. It is not sedating in the way hypnotics are, and there is no dependence or rebound. The one context requiring care is clozapine therapy, where glycine may reduce the drug's effectiveness.
This guide is educational, not medical advice. Talk to a physician or pharmacist before starting any supplement, especially alongside prescription medication or a medical condition.
Evidence review last updated