GlycineAmino Acid

• Sleep after a short nightPossibly helps

  A randomized trial gave 3 g of glycine before bed after three nights of 25%-restricted sleep [5]. Next-day fatigue fell on a visual analog scale, and psychomotor vigilance test performance improved versus placebo.
  
  Plasma melatonin and circadian clock-gene expression did not shift, so whatever is happening is not a melatonin effect [5].
  
  A companion rat study found oral glycine shortened NREM sleep latency and lowered core body temperature via NMDA-receptor vasodilation [6]. That is animal data offering a plausible route, not a human result.
  
  A 2024 systematic review across human organ systems found nervous-system outcomes to be glycine's most consistent human evidence, with metabolic and muscular ones thinner [20]. ^[5]

• Negative symptoms in schizophrenia, without clozapinePossibly helps

  Heresco-Levy and colleagues, in Archives of General Psychiatry, added 0.8 g/kg/day of glycine to conventional or non-clozapine atypical antipsychotics [3].
  
  Negative symptoms fell 30% ± 16% over 6 weeks versus placebo, in patients with enduring negative-symptom schizophrenia [3].
  
  That is a single trial at that dose. Nothing in the sources reviewed here replicates it. ^[3]

• Added on top of clozapineNot shown to help

  Here the direction flips. In patients maintained on clozapine at 400–1,200 mg/day, adding 30 g/day of glycine for 12 weeks moved neither positive nor negative symptoms [8].
  
  The clozapine-only arm reached a 35% reduction in positive symptoms that the glycine arm never matched. The authors concluded glycine may interfere with clozapine's antipsychotic efficacy [8].
  
  This rests on one small trial of 19 to 24 patients and needs replication. It is still the most consequential finding on this page for anyone taking clozapine. ^[8]

• Lean mass in dialysis patientsPossibly helps

  A randomized double-blind crossover trial in malnourished chronic hemodialysis patients gave 14 g/day of oral glycine for 4 months [11].
  
  Fat-free mass index and handgrip strength both rose. An equal dose of branched-chain amino acids lowered fat-free mass index in the same patients [11].
  
  Six of 27 people on glycine reported nausea or vomiting [11]. None of this extends to healthy adults or to athletes. ^[11]

• Blood sugar and metabolic markersPossibly helps

  A 2025 randomized clinical trial in adults with severe obesity found no significant change in HbA1c, fasting glucose, fasting insulin, HOMA-IR or the Matsuda index after two weeks [15].
  
  The same trial did lower plasma triglycerides and the liver enzymes ALT and AST [15].
  
  The sources disagree on what to make of that.
  
  A metabolic review notes that low circulating glycine tracks with obesity, type 2 diabetes and fatty liver disease, and reads supplementation as a plausible lever [9]. The two-week trial did not deliver on the glycemic half of that. ^[15]

• Free-form glycine powderReaching the multi-gram amounts the trials actually used, from 3 g upward .

  Elemental content

  Pure free amino acid, with nothing chelated to it

  Absorption

  Highly water-soluble — a manufacturer data sheet lists about 25 g per 100 mL of water at 25°C [21].
  
  No study compared powder against capsules for absorption or effect. That is a genuine evidence gap, not a settled point [17].

  Best for

  Reaching the multi-gram amounts the trials actually used, from 3 g upward [5].

• Magnesium glycinateDelivering magnesium in a well-tolerated form, not for reaching a studied glycine dose .

  Elemental content

  Magnesium chelated to glycine — a magnesium product first, a glycine one second

  Absorption

  Per WebMD, it raises levels of both magnesium and glycine [17].
  
  The glycine delivered per dose is far smaller than a dedicated glycine serving. Magnesium-glycinate marketing is not glycine evidence.

  Best for

  Delivering magnesium in a well-tolerated form, not for reaching a studied glycine dose [17].

• Collagen and gelatinPeople already taking collagen.

  Elemental content

  Peptide-bound glycine — 26–34% of gelatin's amino acid content by weight, per a USDA AMS technical review [13]

  Absorption

  Bound into peptides rather than free, so the kinetics differ from a glycine powder.
  
  No head-to-head human comparison of the two turned up in this research [16].

  Best for

  People already taking collagen. A 2025 study tested a 3:1:1 glycine-proline-hydroxyproline blend with no placebo arm, so it says nothing about glycine alone [16].

• Nausea, vomiting, loose stools

  The most consistently reported effect at higher doses. Six of 27 hemodialysis patients on 14 g/day reported nausea or vomiting over 4 months [11].
  
  Similar gastrointestinal complaints appear across the 15–60 g/day schizophrenia literature [3]. The 3 g sleep dose sits an order of magnitude below that range. ^[11]

• Liver disease and urea cycle disorders

  Unlike most amino acids, glycine is broken down straight to ammonia rather than being transaminated first [2].
  
  A 2025 metabolic review therefore flags a theoretical hyperammonemia risk at high intakes in people with liver disease or a urea cycle disorder [2]. It is a mechanistic caution, not a documented case series. ^[2]

• Blood pressure — a disputed signal

  INTERMAP, a cross-sectional study of 4,680 adults across the US, UK, Japan and China, linked higher dietary glycine to higher blood pressure [10].
  
  A 2-SD-higher intake (+0.71 g/24h) tracked with 2.0–3.0 mmHg higher systolic pressure. The association ran stronger in Western participants, whose glycine came mainly from meat [10].
  
  This is a dietary correlation, not a supplement trial. No study has tested glycine supplements against antihypertensive drugs, so that interaction question is open rather than answered either way [10]. ^[10]

• Pregnancy and breastfeeding

  Two different questions with two different answers. A stable-isotope trial found a real dietary glycine requirement in late gestation, weeks 31–40 [12].
  
  On supplement safety, WebMD states plainly that the effect on pregnancy or a fetus is not known, and that whether glycine passes into breast milk is not known [17].
  
  Higher physiological need in late pregnancy is documented. Supplement safety in that window is not. ^[12]

• A laboratory nitrosation finding

  The same 2025 review notes that gut bacteria can nitrosate glycine derivatives, which then react with DNA to form carboxymethylguanosine [2].
  
  It is raised inside gastrointestinal-cancer research as a laboratory observation. Nothing in the sources connects it to a clinical outcome in supplement users [2]. ^[2]

• Clozapine

  The most consequential interaction on this page. In patients on clozapine at 400–1,200 mg/day, 30 g/day of glycine for 12 weeks produced no symptom improvement [8].
  
  The placebo arm — clozapine alone — reached a 35% reduction in positive symptoms; the glycine arm did not. The trial authors concluded glycine may interfere with clozapine's antipsychotic efficacy [8].
  
  Glycine added to non-clozapine antipsychotics did the opposite, improving negative symptoms [3]. The drug it is paired with changes the direction of the result, so this is not a class-wide statement about antipsychotics. ^[8]

Collagen-rich animal tissue is the concentrated source. A USDA Agricultural Marketing Service technical review of gelatin puts glycine at 26–34% of its amino acid content by weight, alongside proline at 10–18% [13].

Ordinary high-protein foods — meat, fish, dairy, legumes — are described as good sources, with a typical diet supplying around 2 g/day [7]. Population datasets land a little higher, at 2.7–4.7 g/day depending on the cohort [1].

What counts as a glycine-rich food is partly cultural. In INTERMAP, meat dominated for US and UK participants, grains and rice for Chinese participants, fish and rice for Japanese ones [10].

Before bed, if the goal is sleep. Every human trial that found a sleep benefit dosed 3 g at bedtime rather than at any other point in the day [5,6].

No source pinned down a precise window. The familiar "30 to 60 minutes before bed" figure was not traceable to a primary source here, so it is not stated as fact.

For every other goal, timing is untested. The dialysis trial split 14 g into two doses with no stated rationale, and the schizophrenia trials reported daily totals only [11].