Does taking glycine help you sleep better?

Glycine (3 grams) taken before bedtime improved subjective sleep quality in people with insomnia complaints in a Japanese study. One caveat: that study was conducted by Ajinomoto, the manufacturer of glycine supplements. The exact size of the effect is not reported in the published abstract, and independent replication in larger groups is lacking.

A plausible biological mechanism does exist. Glycine activates inhibitory nerve receptors in the brain that play a role in the sleep-wake cycle. Animal studies also show that oral glycine lowers core body temperature and increases skin blood flow. A falling core body temperature is a well-known step in the process of falling asleep. This mechanistic line of evidence is plausible, but animal results are no guarantee of the same effect in humans.

A small RCT (16 participants) found beneficial effects on sleep onset latency, total sleep time and sleep efficiency, but that involved a combination supplement also containing tryptophan, magnesium, tart cherry and L-theanine. Which component made the difference therefore cannot be determined. Glycine alone cannot be isolated from this study.

Broader reviews mention glycine as a possible supplement for sleep problems, but regard the evidence as limited and explicitly position it as a complement to good sleep habits, not a replacement. How glycine interacts precisely with other sleep-related systems (such as melatonin) remains unclear for the time being.

Safety data do not appear in the available abstracts. For an amino acid that the body produces itself and that is also found in protein-rich foods, a dose of 3 grams is probably low-risk, but the absence of safety information in the source means this cannot be extrapolated to long-term use or specific health conditions.