L-theanine and glycine before bed: do they improve HRV, or just help you fall asleep faster?

Two different compounds, two different jobs

It's easy to lump L-theanine and glycine together because supplement brands sell them as a matched set, usually alongside magnesium or melatonin in a "sleep stack." But they're structurally and mechanistically unrelated:

• L-theanine is an amino acid found almost exclusively in tea leaves (and some mushrooms). It crosses the blood-brain barrier and is associated with increased alpha brain-wave activity — the pattern seen during relaxed, unfocused wakefulness, not during sleep itself. Its best-supported use case is calming, not sedating.
• Glycine is one of the simplest amino acids in the body, used throughout protein synthesis and as an inhibitory neurotransmitter in the spinal cord and brainstem. The sleep-specific mechanism under investigation is different: oral glycine is proposed to lower core body temperature by increasing blood flow to the skin, which may help initiate sleep the same way a warm bath followed by cooling does.

Because the mechanisms don't overlap, there's no obvious reason combining them would produce an additive effect — that would need its own trial, and none exists yet.

What the L-theanine evidence actually shows

The pattern is familiar from other sleep supplements in this series: the claim closest to L-theanine's actual mechanism (calming an aroused nervous system) has the most support, and the claim furthest from it (changing an overnight physiological marker) has the least. If you've seen specific numbers online claiming a large HRV jump from L-theanine — one widely shared figure claims an 18% rise in a multi-week trial — treat it with skepticism; we could not trace it to a peer-reviewed source, and it doesn't match the null result from the better-controlled study above.

What the glycine evidence actually shows

Glycine's human trial base is smaller than melatonin's or magnesium's, concentrated in a handful of Japanese research groups, and focused almost entirely on subjective outcomes plus one physiological mechanism that's mostly been confirmed in animals rather than people:

1. Subjective sleep quality. Placebo-controlled trials in adults with mild, self-reported sleep complaints found about 3 g of glycine before bed improved next-morning ratings of sleep quality and reduced daytime sleepiness, compared with placebo.
2. Sleep onset. At least one trial using overnight sleep recordings (not just questionnaires) found glycine shortened the time it took participants to fall asleep, alongside the same subjective improvements.
3. The temperature mechanism. In animal studies, oral glycine measurably lowered core body temperature while increasing blood flow to the skin — the same heat-dissipation pattern your body naturally uses to initiate sleep. This is a plausible, mechanistically coherent story, but the direct human evidence for it is thin: a 2024 trial of a glycine-rich collagen peptide (not pure glycine) in physically active adults with sleep complaints found no measurable difference in core temperature, hormone levels, inflammation, or sleep quality versus placebo.
4. HRV. No published trial we could find measured HRV alongside glycine supplementation, in either direction.

That last gap matters: glycine's sleep-onset evidence is more consistent than L-theanine's HRV evidence, but "shortens time to fall asleep" and "improves the physiological recovery your HRV tracks" are still two separate claims, and only the first one has real data behind it.

Why "take them together" has no trial behind it

Search for L-theanine and glycine together and you'll mostly find retailer and affiliate content recommending a combined "sleep stack" — often alongside magnesium glycinate — on the logic that hitting GABA signaling, alpha-wave activity, and body temperature simultaneously should work better than any single lever. That's a plausible-sounding story, but plausibility isn't evidence: no randomized trial has tested the L-theanine-plus-glycine combination against either ingredient alone, so there's no way to know whether combining them is additive, redundant, or no different from either one by itself. This is the same reasoning gap that shows up in magnesium's sleep evidence when products stack multiple magnesium forms and call it synergistic — stacking compounds that each have modest individual evidence doesn't automatically produce a larger effect, and only a dedicated trial could show that it does.

Dosing: what the trials actually used

Neither range reflects a confirmed optimal dose — it's simply what the available trials happened to test. There's no evidence that doubling either dose produces a bigger effect, and higher doses haven't been studied as thoroughly for safety in the sleep-specific context.

A reasonable way to think about using either one

1. Match the compound to the actual problem. If the issue is racing thoughts or daytime stress bleeding into bedtime, L-theanine's evidence base (calming an aroused nervous system) is the better fit. If the issue is specifically lying awake unable to fall asleep, glycine's evidence base (sleep onset, via a proposed temperature mechanism) is the better fit.
2. Don't expect either one to move your HRV number. If your goal is a higher recovery score, the stronger-evidenced levers remain the ones covered elsewhere in this series — consistent sleep timing, alcohol and caffeine timing, and training load — rather than either of these two compounds.
3. Test one variable at a time. Because no trial has tested the combination, taking both at once makes it impossible to tell which one (if either) is doing anything for you. A few weeks on L-theanine alone, then a few weeks on glycine alone, comparing your own sleep-onset time and next-morning alertness, is more informative than starting a combined stack blind.
4. Track trends, not single nights. Subjective "I slept better" reports are exactly the outcome most of this evidence is built on, which makes them easy to talk yourself into. Comparing a couple of weeks of actual sleep data against a couple of weeks off is a more honest test than how you feel the morning after one dose.
5. Check with a doctor or pharmacist first if you're pregnant, have a chronic condition, or take other medications — this article covers what the sleep-science evidence shows, not a clinical recommendation.

If you want to know whether it's actually working for you

Both of these compounds are sold on a feeling — calmer, sleepier, more rested — that's easy to attribute to the pill itself even when the real driver is a new bedtime routine, less screen time, or a quieter week. Vita's sleep analysis tracks your sleep onset, efficiency, and night-to-night consistency against your own multi-week trend rather than a single night, which is the only honest way to separate a real effect from a placebo-flavored routine change. Asking Vita's AI coach to compare your trend across a few weeks on either compound versus a few weeks off is a faster way to get an answer than guessing from how you feel the morning after.

The bottom line

L-theanine's clearest evidence is for blunting an acute stress response, with weaker and more mixed support for general anxiety and subjective sleep quality — and the one well-controlled trial measuring HRV directly found no effect. Glycine's clearest evidence is for modestly faster sleep onset and better next-day subjective alertness in people with mild sleep complaints, through a temperature-based mechanism that's better established in animals than humans — and no trial has measured its effect on HRV at all. Nobody has tested the two together, so the popular "stack" rests on mechanism-sounds-plausible reasoning rather than data. If your actual goal is a better overnight HRV number, neither compound currently has the evidence to promise that; if your goal is calming pre-sleep racing thoughts or falling asleep a bit faster, each has a narrow, real claim worth testing on your own data rather than taking on faith.