L-theanine

L-theanine is an amino acid from tea that produces a calm, alert state without sedation. As a supplement it gives small improvements in how people rate their own sleep, and short-term gains in attention, with a smaller, less reliable drop in acute stress — but it does not treat an anxiety disorder, and its most-quoted sleep review was written mostly by tobacco-company employees.

If you want to try it, take 200 mg in the evening for sleep, or 200–400 mg during the day for calm focus. Expect a small effect, and don't use it in place of treatment for an anxiety disorder.

What it is

L-theanine is an amino acid found in tea leaves. It crosses into the brain and promotes relaxation without sedation.^[1] A cup of tea contains only about 5–30 mg, along with caffeine, so the studied doses mean a supplement.^[2] See Tea for tea itself.

Sleep: small effects, all on how people feel

The review most often quoted for L-theanine and sleep covered 13 trials and 550 people and concluded that 200–450 mg a day improves falling asleep and sleep efficiency. It pooled nothing, though, and three of its four authors work for a tobacco company.^[3]

The proper meta-analysis, across 19 articles and 897 people, found small effects, and all of them on people's own ratings rather than measured sleep: on falling asleep, on daytime functioning and on sleep quality. The authors note a lack of studies of L-theanine on its own; many trials used products with other ingredients.^[4] A small manufacturer-funded crossover trial in 30 healthy adults, taking 200 mg a day for four weeks, found slightly faster sleep onset and less sleep disturbance than placebo; stress scores did not clearly beat placebo.^[5]

Anxiety: calmer focus, not a treatment

The anxiety case is weaker than it is usually presented. The only trial in people with generalised anxiety disorder, inside the review most often cited for anxiety, found no benefit on anxiety symptoms.^[6]^[7] A 2026 meta-analysis of 31 trials reached the same conclusion for clinical anxiety, while confirming a real short-term gain in attention and a modest drop in acute stress — though the stress result leaned on studies at high risk of bias.^[8]

So its better-supported use is the everyday one: a calmer, more focused state, not treatment for an anxiety disorder.

With caffeine: the "calm alertness" of tea

In tea, theanine comes with caffeine, and the two are often studied together. A single 200 mg dose of theanine speeds reaction times in pooled trials. Small brain-wave studies show it raising the alpha rhythm linked to relaxed attention and partly blunting caffeine's jitters, which is the "calm alertness" tea drinkers describe — but that evidence comes from a handful of small single-lab studies. Pooled estimates for theanine combined with caffeine are less certain than for theanine alone, and one review found the caffeine dose explained more of the effect than the theanine dose. The 100–200 mg doses these trials use would take several to dozens of cups of tea.

None of this shows that theanine protects the brain over years. That idea is a hypothesis: no human trial has shown that tea's constituents protect the brain long term, and the large study linking tea to lower dementia risk could not tell which part of the cup mattered.

Safety

L-theanine is well tolerated across trials, and the US Food and Drug Administration classes it as generally recognised as safe. In theory it may add to the effect of blood-pressure drugs, so take care if you take several. Data beyond a year are limited but show no harm signal. No dependence or rebound on stopping has been documented, though no trial has tested stopping. Combining it with magnesium hasn't been studied, though neither has known interactions.

What to do

For sleep: 200 mg in the evening, as a modest nudge — not a fix for chronic insomnia, where cognitive behavioural therapy is the treatment that works.
For daytime calm: 200–400 mg a day.
For an anxiety disorder: see a clinician; the trials in that group were null.
If you take blood-pressure drugs, watch for an added drop.

Other options are compared on Sleep supplements.