GradedHealth

Glycine

Last reviewed October 2026 · Based on 7 studies

Bottom line

Glycine before bed may modestly improve sleep quality and next-day fatigue. The evidence is limited to a few tiny, short trials at high risk of bias.

What it helps with

  • Helps you sleep — Benefit, evidence grade C (Limited). Small short randomized trials of 3 g glycine before bed; downgraded for small total sample and high risk of bias.
  • Reduces daytime fatigue and sleepiness — Benefit, evidence grade C (Limited). Two tiny short trials with subjective fatigue outcomes; downgraded for small total sample and high risk of bias.

What it is

Glycine is a non-essential amino acid the body makes and also gets from food; adults synthesize about 45 g per day and take in roughly 3–5 g from the daily diet [1]. It acts in the nervous system through glycine receptors and NMDA-type glutamate receptors [2]. Supplement trials for sleep used about 3 g of glycine powder taken 30–60 minutes before bed [3][1].

How it might work

Researchers propose that glycine taken before bed lowers core body temperature and increases blood flow in the skin, a pattern linked with falling asleep, acting through NMDA receptors in the brain's circadian control centre [2][3]. Glycine did not change melatonin levels or clock-gene expression in rats, so it does not seem to reset the circadian clock itself [1].

What the research says

A 2024 systematic review of glycine in adults covered 18 healthy and 34 diseased populations and found sleep benefits in three small healthy-volunteer trials using 3 g per day before bed over 2–4 days: better sleep quality, alertness and cognition, with less fatigue and sleepiness [3]. A randomized single-blind crossover trial in 7 healthy men with sleep restricted by 25% found 3 g of glycine before bed for three nights reduced fatigue and improved vigilance scores the next day [1]. A randomized crossover trial in 13 active men with sleep complaints found glycine-rich collagen peptides (15 g before bed for 7 nights) cut night awakenings without changing total sleep, latency, or efficiency [4].

Doses used in studies

Sleep trials of pure glycine used 3 g taken 30–60 minutes before bed for 2–4 nights [3][1]. The collagen trial used 15 g of glycine-rich collagen peptides 1 hour before bed for 7 nights, which is not the same as 3 g of pure glycine [4]. Much higher single doses (0.8 g per kg of body weight by mouth) worsened sensorimotor gating and cognitive scores in healthy volunteers [3].

Safety and interactions

Short trials of 3 g of glycine for a few nights did not report serious harms [1][3]. Doses far above sleep-trial doses have shown negative effects on thinking and sensorimotor tests, and one review notes possible cell toxicity above 500 mg per kg of body mass [3][5]. No data were found in these reviews on pregnancy and breastfeeding, on interactions with sleep or psychiatric medicines, or on a formal upper intake limit for glycine [3][6]. People who are pregnant, breastfeeding, or taking sleep or psychiatric medicines should talk to a doctor first [3].

Caveats and open questions

The sleep trials were tiny (about 7–13 people each) and lasted only days, and the review judged them at overall high risk of bias, with no low-bias trials in healthy populations [3][4]. Most outcomes were subjective ratings rather than diagnosed insomnia or long-term sleep health [3][1]. Multi-ingredient blends containing glycine cannot show what glycine alone does [7].

References

  1. The effects of glycine on subjective daytime performance in partially sleep-restricted healthy volunteers.. Front Neurol, 2012.
  2. New therapeutic strategy for amino acid medicine: glycine improves the quality of sleep.. J Pharmacol Sci, 2012.
  3. The effect of glycine administration on the characteristics of physiological systems in human adults: A systematic review.. Geroscience, 2024.
  4. Collagen peptide supplementation before bedtime reduces sleep fragmentation and improves cognitive function in physically active males with sleep complaints.. Eur J Nutr, 2024.
  5. An Update of the Promise of Glycine Supplementation for Enhancing Physical Performance and Recovery.. Sports (Basel), 2024.
  6. Multifarious Beneficial Effect of Nonessential Amino Acid, Glycine: A Review.. Oxid Med Cell Longev, 2017.
  7. Nutritional Modulation of Sleep Latency, Duration, and Efficiency: A Randomized, Repeated-Measures, Double-Blind Deception Study.. Med Sci Sports Exerc, 2023.