In plain terms: Glycine shows early signals in human research but the evidence is limited -- small trials, surrogate endpoints, or not yet independently replicated. It is an emerging area, not yet a proven one.
Glycine
The evidence as the record states it
Bannai et al. (Frontiers in Neurology, 2012; PMID 22529837): 3 g of glycine before bed against placebo in partially sleep-restricted healthy volunteers significantly reduced next-day fatigue on a visual analogue scale and improved psychomotor vigilance, with a non-significant trend on sleepiness. This is the only placebo-controlled human study of glycine ALONE that bears on sleep, it is small, it enrolled healthy volunteers under experimental sleep restriction rather than people with insomnia, and PubMed assigns it no trial publication type. The two GlyNAC papers below tested glycine COMBINED with N-acetylcysteine and cannot support a grade for glycine on its own: Kumar et al. (Journal of Gerontology, 2023; PMID 35975308) and Wang et al. (Frontiers in Nutrition, 2026; PMID 42232577). Graded C on that basis. The frequently cited Yamadera 2007 glycine sleep study was published in Sleep and Biological Rhythms, which PubMed does not index, so it carries no identifier and is not used here.
Why Grade C and not Grade B
Grade C reflects early or limited human evidence -- small trials, surrogate endpoints, or preliminary findings not yet independently replicated. The evidence points in a promising direction but does not yet meet the standard for Grade B. A single well-powered, independently replicated trial with a meaningful endpoint would move this to Grade B or higher.
What would move this grade
A well-powered randomized trial with an objective, clinically meaningful primary endpoint and at least one independent replication would support Grade B or higher.