Glycine

Dietary supplement

What it is

Glycine is the smallest amino acid, and in the nervous system it doubles as an inhibitory neurotransmitter. Taken before bed at 3 g, it slightly lowers core body temperature — one of the physiological signals associated with sleep onset. That mechanism, plus its sweet taste and low cost, is why it shows up in evening routines.

The trial evidence deserves a plain description: a handful of small studies, all from a single research group, showing quicker arrival at stable sleep, better rated sleep quality, and less next-day sleepiness at 3 g. Promising and consistent — but small, and without independent replication. That is a grade C, and we present it as exactly that.

Evidence by goal

Sleep

At 3 g before bed: quicker arrival at stable sleep on polysomnography, better rated sleep quality, and less next-day sleepiness during restricted sleep — small trials from one research group, replication pending.

Yamadera W, 2007 · Inagawa K, 2006 · Bannai M, 2012

Forms compared

Powder. Sweet-tasting and dissolves easily — the cheapest way to reach 3 g. Stir into water or evening tea.
Capsule. Convenient, but 3 g means swallowing several large capsules. Powder is the practical form at this dose.

Dose and timing

3 g, 30–60 minutes before bed — the dose every trial used. Going up to 5 g stays within the studied range for tolerability, but there is no evidence that more works better for sleep.

Evidence-backed range: 3–5 g per day.

Cautions and interactions

Well tolerated in trials at these doses; mild stomach softness is the most reported effect.
The evidence base is small and comes from a single research group — hold your expectations to grade C.
If you take prescription sleep medication, bring any added sleep aid to your prescriber rather than stacking on your own.

Who should skip this

People expecting a sedative — glycine nudges sleep physiology; it does not knock anyone out.
People who want only well-replicated evidence behind what they take — this one is not there yet.

Questions we actually get

How does glycine relate to sleep at all?

It acts as an inhibitory neurotransmitter and slightly lowers core body temperature when taken before bed — a signal associated with falling asleep. The trials measured quicker arrival at stable sleep at 3 g.

What does it taste like?

Mildly sweet — close to sugar at a fraction of the sweetness. Most people find the powder easy to take in water.

Can I combine it with magnesium or melatonin?

Start with one at a time so you can tell what is doing what. If you take prescription sleep medication, involve your prescriber before adding anything.

Why only grade C when the results sound good?

Because all the trials are small and come from one research group. Consistent small trials without independent replication earn a C on our rubric — no more, no less.

Sources

Last reviewed: pending review