L-theanine: what the tea amino acid has, and hasn't, been shown to do
A cup of tea holds a small amount of it. Supplements hold ten times more. Between those two sits a modest pile of human studies, several funded by the companies that make it. Here is what they found, read without the gloss.
contentsWhat L-theanine is
Key points
- L-theanine is an amino acid found in tea. A regular cup holds roughly 4.5 to 22.5 mg.
- Studies used far more: about 97 to 200 mg in a single serving.
- In blood it peaks around 50 minutes after drinking and halves roughly every hour.
- A 2025 review found small improvements in how people rated their sleep. Paired with caffeine, it helped some attention tasks in lab tests.
- In the one trial we read that measured it, cortisol did not change. Several studies were funded by companies that make or sell it.
What L-theanine is
L-theanine is an amino acid found naturally in tea leaves. It is one reason tea is often described as calmer than coffee, even though both contain caffeine. You will see it sold on its own, in focus blends with caffeine, and in night-time formulas.
The research on it is real but modest. Most studies are small, short and run on healthy young adults. This article walks through what they actually tested, and points to our closer looks at sleep and the pairing with caffeine.
A cup versus a capsule
One lab study reported that a regular cup of tea holds about 4.5 to 22.5 mg of L-theanine.[40] The amounts tested in studies sit well above that.
chart · milligrams of L-theanine per serving
Study doses were several cups' worth or more
A cup of tea, up to
22.5 mgFocus study with caffeine (2010)
97 mgFocus study with caffeine (2008)
100 mgNightly sleep trial (2019)
200 mg
That gap matters when you read headlines. A finding at 200 mg tells you little about what two cups of green tea do.
How long it lasts
In a small study of 15 young men, L-theanine from tea or from water reached its highest level in the blood about 50 minutes after drinking. Its elimination half-life was about 65 minutes.[41]
Put simply, it arrives within the hour and most of it has cleared from the blood within a few hours. Blood levels are not the same as brain levels, and the study was small and unblinded, so treat those numbers as a rough guide. It does help explain why focus studies tested people within an hour or two of a dose.
What the human studies tested
table · the main questions so far
| Question | What was found | How strong |
|---|---|---|
| Sleep[38] | Small gains in self-rated sleep onset, daytime functioning and overall sleep quality across 19 trials | Modest. Few trials tested L-theanine on its own |
| Focus, with caffeine[40][39] | Better accuracy when switching between tasks, in single-dose lab sessions | Two small, industry-funded studies |
| Stress symptoms[37] | Stress scores improved from the start of the trial, but not compared with placebo | One 30-person crossover trial |
| Cortisol[37] | No change in saliva or blood cortisol after 4 weeks | Same trial |
Does it lower cortisol?
On the evidence we read, no. The 4-week trial that gave 30 healthy adults 200 mg each night measured cortisol in saliva and blood. Neither changed.[37]
That is one trial, and a small one, so it does not close the question. But claims that L-theanine “lowers cortisol” are running ahead of the best data we found. For what does shift cortisol across a day, see our piece on the detox trend.
Who paid for the research
It is worth knowing who ran these studies. The nightly sleep trial was funded by an L-theanine manufacturer, and two of its authors worked there.[37] Both caffeine studies and the blood-level study had authors employed by tea companies.[39][40][41]
Industry funding does not make a result wrong. It is a reason to want independent studies that repeat it, and right now there are not many.
Where that leaves you
L-theanine looks well studied for a narrow set of questions and thinly studied for the bigger ones. If you are curious, tea is the low-stakes way to meet it. If your nights are the real problem, the sleep calculator and a week in a sleep diary are better first steps than any capsule.
One ingredient. Every study. Plain English.
Subscribers get our next ingredient breakdown first, with the funding notes included.
Sources
- Giesbrecht et al., 2010. The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness. Nutritional Neuroscience.
- Hidese et al., 2019. Effects of L-theanine administration on stress-related symptoms and cognitive functions in healthy adults: a randomized controlled trial. Nutrients.
- Owen et al., 2008. The combined effects of L-theanine and caffeine on cognitive performance and mood. Nutritional Neuroscience.
- van der Pijl et al., 2010. Human disposition of L-theanine in tea or aqueous solution. Journal of Functional Foods.
- Bulman et al., 2025. The effects of L-theanine consumption on sleep outcomes: a systematic review and meta-analysis. Sleep Medicine Reviews.
This article is educational and isn’t medical advice. It was written and checked by the UNWIRE™ editorial team; no clinician has reviewed it yet. UNWIRE™ is a nutrition company, and this article does not describe any product.
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