Glycine
TL;DR
The simplest amino acid — but don't let that fool you, it's remarkably versatile
One of the best-evidenced supplements for sleep onset — works the SAME NIGHT
Lowers core body temperature within 30-60 minutes, triggering sleep onset
Also a building block for glutathione, collagen, and creatine
Essentially zero side effects — your body makes it naturally
Absurdly cheap: ~$5-8/month as bulk powder
Best for: Sleep onset, sleep quality, general foundational supplement
What it is
Glycine is the simplest amino acid — just a hydrogen atom as its side chain. But this simplicity is deceptive. Glycine is involved in a remarkable number of biological processes: it's a building block for collagen (the most abundant protein in your body), glutathione (the most important antioxidant), creatine (critical for energy), heme (oxygen transport), and DNA bases.
In the brain, glycine serves as both an inhibitory neurotransmitter (via glycine receptors, promoting calm) and a co-agonist at NMDA receptors (involved in learning and memory). This dual role is unusual and gives glycine a unique neurological profile.
For sleep specifically, glycine works through an elegant mechanism: it acts on NMDA receptors in the suprachiasmatic nucleus (your brain's master clock), which triggers vasodilation in the extremities. This moves blood from your core to your hands and feet, dropping core body temperature — which is one of the primary physiological triggers for sleep onset. Your body does this naturally every evening; glycine just amplifies the signal.
What the research says
Glycine's sleep evidence is unusually clean and compelling for a supplement.
The key studies. A series of studies by Bannai, Kawai, and colleagues at Ajinomoto (a major amino acid manufacturer) demonstrated that 3g of glycine taken before bed significantly reduced sleep latency and improved subjective sleep quality. Importantly, participants also showed improved next-day cognitive performance — suggesting genuinely better sleep, not just sedation.
Speed of onset. This is glycine's standout feature among sleep supplements. The core body temperature drop begins within 30-60 minutes of ingestion. Most users report effects on the very first night. Compare this to valerian (2-4 weeks) or magnesium (days to a week).
Objective measures. Studies using polysomnography (sleep lab monitoring) showed that glycine increased time spent in slow-wave (deep) sleep and reduced time to reach it. This isn't just 'people felt like they slept better' — the brain waves confirm it.
No hangover effect. Unlike many sleep aids (including higher-dose melatonin), glycine does not cause next-morning grogginess. In fact, next-day alertness and cognitive function improved in studies — likely because of genuinely better sleep architecture.
The combination opportunity. Glycine pairs naturally with magnesium glycinate (which already contains glycine) and taurine. These three together target different aspects of the sleep initiation process: temperature (glycine), muscle relaxation (magnesium), and neural calming (all three).
Glycine has emerging but limited evidence for metabolic health.
Insulin sensitivity. Some evidence suggests glycine improves insulin sensitivity, possibly through its role in glutathione synthesis (reducing oxidative stress that impairs insulin signaling) and through direct effects on inflammatory pathways.
Glutathione production. Glycine is a rate-limiting precursor for glutathione synthesis along with cysteine (from NAC) and glutamate. A study by Sekhar et al. showed that supplementing glycine and NAC together dramatically restored glutathione levels in older adults.
Body composition. Very limited evidence for direct metabolic effects. Glycine's primary metabolic benefit is likely indirect through improved sleep — since poor sleep significantly worsens insulin resistance.
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Safety
Glycine has essentially a perfect safety profile. It's an amino acid your body produces naturally and that's abundant in protein-rich foods. There is no established upper limit for supplemental glycine.
At the standard sleep dose of 3g, no adverse effects have been reported in clinical trials. Even at significantly higher doses (up to 30g/day in some research contexts), glycine is well-tolerated.
The only notable 'side effect' is the sweet taste — glycine is actually used as a sweetener in some applications. This makes the powder form pleasant to take dissolved in water.
There are no populations for whom glycine supplementation at standard doses raises safety concerns.
Interactions
Glycine has the cleanest interaction profile of virtually any supplement:
- Other sleep supplements: Additive calming effects with GABA, taurine, and magnesium. This is generally desirable — start with lower doses of each when combining.
- Clozapine: Theoretical interaction — glycine may affect clozapine's efficacy. Relevant only for schizophrenia patients on this specific medication.
No CYP enzyme interactions. No interactions with common medications.
Dosing
For sleep: 3g taken 30-60 minutes before bed. This is the dose used in the clinical trials. No need to work up to it — 3g from the first night is appropriate.
Form: Bulk powder is the most economical and practical form. Glycine dissolves easily in water and tastes slightly sweet. Capsules work but you'd need 4-6 capsules to hit 3g, making powder more convenient.
Timing: Before bed for sleep. The temperature-dropping effect begins within 30-60 minutes.
Can you get enough from food? Bone broth is rich in glycine (collagen is one-third glycine by composition), but you'd need a lot to hit 3g of free glycine. Supplementation is more reliable.
Cost
Glycine is absurdly affordable. Bulk powder costs $5-8/month even at 3g/day. A one-pound bag typically lasts 2-3 months.
There is no reason to buy expensive branded glycine products. Pure glycine is pure glycine — the molecule is identical regardless of manufacturer. Buy the cheapest USP-grade powder you can find.
The bottom line
Glycine is the rare supplement where the evidence is clean, the safety is perfect, the cost is trivial, and the effects are noticeable on the first night. If you have any sleep onset issues and you haven't tried 3g of glycine powder before bed, it's one of the easiest experiments in the supplement world. The sweet taste, the fast onset, and the lack of next-day grogginess make it unusually user-friendly.
Sources for this page include published meta-analyses, systematic reviews, and NIH dietary supplement fact sheets. All claims reflect the evidence as of early 2026.
This is not medical advice. Consult your healthcare provider before starting any supplement, especially if you take medications.