Glycine vs Vitamin D for sleep: which has better evidence?
A tie on evidence grade for sleep (Glycine Grade C vs Vitamin D Grade C). For sleep, the two are evidence-matched; the difference is in effect and safety.
Platform evidence comparison
These existing platform assessments are separate from the source-reported findings below. They do not represent a direct trial comparing these treatments.
| Glycine | Vitamin D | |
|---|---|---|
| Grade | grade C | grade C |
| Certainty | low | low |
| Effect | small trials suggest improved subjective sleep quality and reduced next-day fatigue | Systematic review and meta-analysis (moderate certainty): vitamin D supplementation showed promise for improving sleep quality, but its effect on sleep quantity, sleep disorders, sleepiness, and restless legs syndrome was inconsistent across studies. Promising but not settled. |
| Studied dose | 3 g before bed | varies across trials |
| Population | adults | adults |
| Safety | 0 modeled interaction(s) | 1 modeled interaction(s) |
Grade C / low: hints of benefit in small trials; a well-run RCT could easily push this to B or to null.
Direct comparison evidence
There are no direct RCTs in our corpus comparing Glycine and Vitamin D head-to-head for sleep. What we present above is an indirect comparison: each side's own trials, contrasted. This is standard practice for supplement comparisons — dose, population, and outcome-instrument differences between the two evidence bases add uncertainty on top of each side's own grade.
Can you take both?
No modeled interaction between Glycine and Vitamin D in our corpus. Absence of a recorded interaction is not proof of additive benefit — combined-effect trials are usually absent for supplement pairs, so treat "stack" claims skeptically.
Decision guidance
- If certainty matters most — choose whichever side carries the higher certainty rating at a similar grade.
- If you're on medications — check each side's interaction pages before deciding; safety differences dominate small evidence-grade gaps.
- If your population is under-studied — the smaller-effect side with better population evidence often wins over the higher-grade side studied in a different demographic.
- If cost matters — form and dose page differences (Glycine vs Vitamin D) can invert the practical winner.
Grade history
No grade changes yet. Initial grade assigned; re-reviewed on every 6 months (Grade C+) — next review by . Methodology v1.
Re-review cadence: every 6 months (Grade C+) · next scheduled by · methodology v1.
Frequently asked
Which has stronger evidence: Glycine or Vitamin D for sleep?
Glycine carries Grade C at low certainty; Vitamin D carries Grade C at low. The stronger evidence points to a tie, subject to the caveat that direct head-to-head trials are usually absent for supplement pairings.
Can I take Glycine and Vitamin D together?
No modeled interaction between Glycine and Vitamin D; combining is not documented to be problematic, but combined-effect trials are also absent, so treat additive benefit claims skeptically.
Have Glycine and Vitamin D been compared head-to-head?
Direct RCTs comparing the two for sleep are uncommon. This page contrasts each side's own trial evidence — that is an indirect comparison, which introduces confounds from population and dose differences. See the direct-comparison block above.
What decides between Glycine and Vitamin D for me?
Read the decision guidance below — the split usually comes down to population fit, interaction risk with your other medications, and how much certainty you want behind the recommendation.
Related
Cite this page
Citation is encouraged. The proposed reuse license is CC-BY-4.0; status: pending-governance-ratification.
Plain-text citation
Substrate. Glycine vs Vitamin D for sleep. https://evidencebased.info/compare/glycine-vs-vitamin-d-for-sleep. Updated 2026-07-14.
BibTeX
@misc{substrate_nevsvitamindforsleep,
author = {Substrate editorial},
title = {Glycine vs Vitamin D for sleep},
year = {2026},
url = {https://evidencebased.info/compare/glycine-vs-vitamin-d-for-sleep},
note = {evidencebased.info}
}