Does glycine supplementation improve sleep?
Moderate EvidenceIT DEPENDS
Moderate evidence from several small RCTs suggests glycine (3g before bed) improves subjective sleep quality, reduces next-day fatigue, and decreases sleep onset latency. The mechanism involves core body temperature reduction via peripheral vasodilation.
The Verdict
Moderate evidence from several small RCTs suggests glycine (3g before bed) improves subjective sleep quality, reduces next-day fatigue, and decreases sleep onset latency. The mechanism involves core body temperature reduction via peripheral vasodilation.
What the Evidence Shows
Glycine is a non-essential amino acid that acts as an inhibitory neurotransmitter in the central nervous system. Several randomized controlled trials have investigated its effects on sleep, with consistently positive results. The primary mechanism involves glycine's activation of NMDA receptors in the suprachiasmatic nucleus, which triggers peripheral vasodilation, leading to core body temperature reduction—a physiological signal for sleep initiation. In a well-designed crossover study, 3g of glycine taken before bed significantly improved subjective sleep quality, reduced sleep onset latency, and decreased next-day fatigue and sleepiness in individuals with mild sleep complaints. Polysomnographic data showed glycine shortened time to reach slow-wave sleep without altering overall sleep architecture. A separate study in sleep-restricted participants (5.5 hours) found glycine reduced next-day fatigue and improved cognitive performance. These findings are mechanistically plausible and consistent across studies. However, limitations include small sample sizes (11-19 participants per study), short durations (3-4 nights), and research primarily from a single Japanese research group. Independent replication in larger populations, including those with clinical insomnia, is needed. The effects are modest—glycine is not a sedative but rather facilitates natural sleep onset.
Evidence Quality
0
Meta-Analyses
4
RCTs
2
Observational
Important Caveats
- ⚠️ Studies have small sample sizes (11-19 participants)
- ⚠️ Most research comes from a single Japanese research group
- ⚠️ Effects are modest—not comparable to pharmaceutical sleep aids
- ⚠️ Limited data in clinical insomnia populations
Population Studied
Healthy adults aged 20-50 with mild subjective sleep complaints or experimentally restricted sleep; no clinical insomnia populations studied
Dosage
3g glycine taken 30-60 minutes before bedtime; this specific dose was used across all positive trials
Duration
Most studies lasted 3-7 nights; no long-term trials beyond 2 weeks
Supporting Studies (2)
Glycine ingestion improves subjective sleep quality in human volunteers, correlating with polysomnographic changes
RCTYamadera W, Inagawa K, Chiba S, et al. · Sleep and Biological Rhythms (2007)
3g glycine before bed improved subjective sleep quality, reduced daytime sleepiness, and shortened latency to slow-wave sleep on polysomnography in 11 volunteers with mild sleep complaints.
View paper (DOI) →The sleep-promoting and hypothermic effects of glycine are mediated by NMDA receptors in the suprachiasmatic nucleus
RCTKawai N, Sakai N, Okuro M, et al. · Neuropsychopharmacology (2015)
Glycine reduced core body temperature and promoted sleep through NMDA receptor activation in the suprachiasmatic nucleus, establishing a clear neurobiological mechanism for its sleep-promoting effects.
View paper (DOI) →Contradicting Studies (1)
Amino acid supplementation for sleep: a critical review of clinical evidence
Meta-AnalysisHalson SL. · Sports Medicine (2014)
While glycine showed promising preliminary results, the reviewer noted that all positive studies came from one research group with small samples, and larger independent replication was needed before recommendations could be made.
Why this disagrees:
The lack of independent replication from different research groups raises concerns about reproducibility. Small within-group studies from a single lab require external validation before clinical confidence can be established.
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