L-Theanine and Glycine Together for Sleep: What the Research Covers

TL;DR

• No trial has tested L-theanine and glycine as a pair, on their own, for sleep.
• Each has been studied alone. Glycine at 3 g before bed. L-theanine at 200 to 450 mg a day.
• One 2026 trial gave young athletes a blend that included both, plus other ingredients. It found no meaningful effect on sleep over two nights.
• There's no known reason the two would clash. They were studied for different things: glycine for body temperature, L-theanine for stress.
• Neither has an authorised EU health claim.

People pair these two because they show up on most lists of melatonin-free sleep options and both are amino acids. That logic is reasonable. The research behind the pairing is thin, and it's worth knowing exactly how thin before you buy a product that combines them.

What glycine was tested for

Glycine is the simplest amino acid. Your body makes it, and you also get it from protein foods, especially collagen-rich ones. The sleep research centres on body temperature. After a 3 g evening dose, studies measured a small fall in core temperature, linked to more blood flow to the hands and feet (Bannai and Kawai 2012).

The best known human trial took healthy volunteers who cut their sleep by a quarter for three nights. With 3 g of glycine before bed they reported less fatigue the next day than on placebo (Bannai 2012). A 2026 scoping review found three glycine sleep trials in total. All three reported a benefit on at least one sleep outcome, and all three were small (Fan 2026).

What L-theanine was tested for

L-theanine comes from tea. Most of its research is on stress and the body's stress response. For sleep, a 2025 systematic review found 13 trials with 550 participants, at doses from 50 to 900 mg a day. The authors concluded that 200 to 450 mg a day was associated with better sleep measures in adults, mostly on how people rated their sleep (Cotter 2025). Several of the authors work in industry.

L-theanine isn't a sedative. Reviewers describe it as lowering arousal without daytime drowsiness.

The one trial that included both

A 2026 trial at a football academy gave 12 male players aged around 16 a sleep blend after matches. The blend contained 3,000 mg of glycine, 200 mg of L-theanine and 300 mg of magnesium, along with cherry juice concentrate, cherry extract, cocoa extract, tryptophan and 5-HTP. Each player did the blend and the placebo twice, two nights each time. Sleep was measured with wrist actigraphy. The researchers found only trivial differences from placebo on time asleep, time to fall asleep, wake after sleep onset and sleep efficiency (Jackson 2026).

That trial has limits. Twelve players is a small group. Teenage athletes after a match are a narrow population who may already sleep fairly well once they're down. Two nights is short. And with eight ingredients, it can't tell you anything about glycine and L-theanine as a pair.

It still matters. It's the closest thing to a test of the combination, and it didn't show an effect.

Taking them together

Nothing in the research suggests the two interact badly. Both are amino acids found in food, and both were well tolerated at the doses studied. If you want to try them together, these are the doses the single-ingredient trials used:

	Glycine	L-theanine
Dose studied	3 g	200 to 450 mg a day
Timing in trials	Shortly before bed	Varied, often evening
What was measured	Next-day fatigue, sleep ratings, core temperature	Sleep questionnaires, stress measures
Number of sleep trials	Three, all small	Thirteen in one review

A practical approach is to add one at a time. Try one for two weeks, then add the other. That way you can tell whether either is doing anything for you.

Check with a doctor or pharmacist first if you take medication for blood pressure, mood or sleep, or if you're pregnant or breastfeeding. The trials didn't include those groups.

Frequently asked questions

Can you take L-theanine and glycine together?

There's no known reason they would clash, and both were well tolerated in their own trials. No trial has tested the pair alone. The one blend that included both, alongside six other ingredients, made no meaningful difference to sleep in young athletes (Jackson 2026).

What dose of L-theanine and glycine is used for sleep?

Glycine trials used 3 g before bed. L-theanine trials mostly used 200 to 450 mg a day (Cotter 2025).

Is the combination better than either one alone?

Nobody has measured that. It's a reasonable guess that two compounds studied for different things might add together, and it's still a guess.

When should I take them?

Glycine was given shortly before bed in the trials. L-theanine timing varied. Taking both in the hour before bed is the simplest routine.

Disclaimer

This article is for general information only. It is not medical advice and it does not replace guidance from a qualified healthcare professional. Neither glycine nor L-theanine has an authorised EU health claim. Food supplements are not a substitute for a varied diet and a healthy lifestyle. If you are pregnant, breastfeeding, taking medication or living with a health condition, speak with a qualified healthcare professional before using any supplement.