The short version
- In a randomised trial in sleep-restricted healthy volunteers, 3 g of glycine before bed significantly reduced next-day fatigue and improved psychomotor vigilance, while the reduction in sleepiness only showed a non-significant trend.
- Rat work published in Neuropsychopharmacology found glycine promotes non-REM sleep by raising cutaneous blood flow and lowering core temperature via NMDA receptors in the suprachiasmatic nucleus; ablating that nucleus abolished the effect.
- Nearly all the human glycine sleep research comes from the Frontier Research Laboratories at Ajinomoto, an amino acid manufacturer, and has not been independently replicated at scale.
- An independent crossover trial in 13 active men found 15 g of glycine-rich collagen peptides before bed reduced polysomnographic awakenings from 29.3 to 21.3, with no change in sleep latency, efficiency or total sleep time.
- LUVO does not sell glycine and none of its sleep formulas contain it.
Glycine is the smallest amino acid there is, it costs almost nothing, and it turns up in bedtime stacks with a confidence that outruns its evidence base by some distance. The interesting part is that the evidence is not bad. It is small, consistent, and almost entirely produced by one company.
What the human data actually shows
Bannai and colleagues published the most-cited human result in Frontiers in Neurology in 2012. Healthy volunteers had their sleep restricted to 25% less than usual for three consecutive nights, and took either 3 g of glycine or placebo before bed. On the following day, the glycine group showed a significant reduction in fatigue on a visual analogue scale, a tendency toward reduced sleepiness that did not itself reach significance, and a significant improvement on the psychomotor vigilance test.
Read that carefully, because the framing matters. That trial did not measure whether glycine improved sleep. It measured whether people functioned better the next day after a short night. Fatigue moved. Sleepiness only trended. Vigilance improved. Those are next-day outcomes, and they are the outcomes most sleep supplements never bother to measure at all.
The companion review in the Journal of Pharmacological Sciences describes the earlier human work, in which glycine ingestion before bedtime significantly improved subjective sleep quality in people with insomniac tendencies. Both papers come from the Frontier Research Laboratories at Ajinomoto — an amino acid manufacturer. We are pointing that out rather than burying it. It does not make the results wrong. It does mean the field has not been independently replicated at any scale, and you should discount accordingly.
The cooling mechanism is the genuinely good part
Kawai and colleagues, working with Stanford's sleep laboratory, published the mechanistic study in Neuropsychopharmacology in 2015. In rats, oral glycine induced non-REM sleep and shortened the latency to non-REM sleep, with a simultaneous fall in core temperature. Injected into the brain, it raised cutaneous blood flow at the plantar surface in a dose-dependent way — the animal dumped heat through its feet. Blocking NMDA receptors prevented that effect; blocking glycine receptors did not. Microinjection into the suprachiasmatic nucleus reproduced it, and ablating the suprachiasmatic nucleus abolished the sleep-promoting and hypothermic effects entirely.
That is a specific, testable route rather than a vague "calming" story: glycine appears to promote sleep by triggering peripheral vasodilation and heat loss through NMDA receptors in the body clock. Core temperature falling is one of the reliable physiological signatures of sleep onset. A supplement that nudges that signal is doing something coherent.
It is rat data. We are not going to pretend otherwise. But it is unusually clean rat data, and it explains why the human effect, when it appears, shows up in how quickly people settle rather than in how many total hours they log.
The independent trial that got closest
The most interesting recent evidence did not set out to test glycine at all. Thomas and colleagues gave 13 physically active men with sleep complaints 15 g per day of glycine-rich collagen peptides an hour before bed for seven nights, in a randomised crossover design published in the European Journal of Nutrition. Polysomnography on night seven showed fewer awakenings with collagen than control — 21.3 versus 29.3 counts — and better accuracy on a Stroop test the next morning. There were no differences in sleep quantity, latency, efficiency, core temperature, or subjective sleep quality.
So: fewer awakenings, no change in how fast they fell asleep, and none of the temperature effect the rat work would predict. Thirteen men. The supplements were supplied by the funder and two authors were employees of it. That is the honest state of the independent evidence — one small crossover in a population that is not most people.
The part where we talk ourselves out of a sale
We do not sell glycine. None of our sleep formulas contain it, and we are not going to reformulate one on the strength of this literature in order to have something to link here. If you want to test glycine, you will buy it from someone else, probably for a few pence a night, and that is a fine outcome.
What we would say is that glycine belongs in the same honest tier as valerian and tart cherry juice: small trials, a plausible mechanism, an effect that is real in direction and modest in size, and nowhere near enough evidence to justify a confident claim. If a brand tells you glycine will transform your sleep, ask them which trial they are describing, and then ask who paid for it.
What we'd actually tell you
If you try it, 3 g before bed is the dose that has actually been studied, and it is the one worth copying rather than improvising upward. Judge it on the next day rather than the night — fatigue and daytime function are the outcomes with the clearest signal, which is unusual and, frankly, more useful than another fifteen minutes of nominal sleep time.
Give it two weeks. It is well tolerated and cheap enough that the cost of finding out is close to nothing. And as always: if the problem is that you snore, wake gasping, or feel unrefreshed regardless of hours in bed, no amino acid is the answer to that question and a clinician is.
Good questions
How much glycine should I take before bed?
Three grams, taken shortly before bedtime, is the dose the human trials actually used. There is no evidence that more works better, and improvising upward puts you outside the studied range for no known gain. Glycine is cheap and well tolerated, so the cost of a two-week test is low. Judge it on next-day fatigue rather than on hours slept.
Does LUVO sell a glycine supplement?
No. None of our sleep formulas contain glycine and we are not going to add it just so this article has something to link to. If you want to test glycine you will buy it elsewhere, likely for a few pence a night. Magnesium glycinate contains glycine bound to magnesium, but it is not a 3 g glycine dose and should not be treated as one.
Why does glycine make you sleepy?
The best evidence points to body temperature. In rats, glycine increased blood flow to the skin and lowered core temperature, and blocking NMDA receptors in the suprachiasmatic nucleus stopped the effect. Core temperature falling is a normal signature of sleep onset. That is a coherent mechanism, but it is animal data, and the human work has not confirmed a temperature change.
Is the glycine research trustworthy given who funded it?
Treat it with caution. The main human trials come from an amino acid manufacturer's own laboratory, and the one independent-ish study used collagen peptides supplied by the funder with two authors employed by it. Industry funding does not make results false, but unreplicated industry results deserve a discount. Nobody has run a large independent trial of glycine for sleep.
Is glycine better than melatonin for sleep?
Nobody has compared them head to head, so any answer is a guess. They also do different things: melatonin is a timing signal, while glycine appears to work through heat loss and next-day recovery. The glycine evidence base is smaller and more conflicted by funding. If you want the better-studied option, it is not glycine.
Sources
- Bannai M, Kawai N, Ono K, Nakahara K, Murakami N. The effects of glycine on subjective daytime performance in partially sleep-restricted healthy volunteers. Front Neurol, 2012. View study
- Bannai M, Kawai N. New therapeutic strategy for amino acid medicine: glycine improves the quality of sleep. J Pharmacol Sci, 2012. View study
- Kawai N, Sakai N, Okuro M, et al. The sleep-promoting and hypothermic effects of glycine are mediated by NMDA receptors in the suprachiasmatic nucleus. Neuropsychopharmacology, 2015. View study
- Thomas C, Kingshott RN, Allott KM, et al. Collagen peptide supplementation before bedtime reduces sleep fragmentation and improves cognitive function in physically active males with sleep complaints. Eur J Nutr, 2024. View study
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
