Is L-theanine good for stress and sleep?

L-theanine, an amino acid that occurs naturally in green tea, is being investigated as a remedy for stress and sleep problems. The most direct evidence for both applications together comes from a single small double-blind randomised study (30 participants, 200 mg per day for four weeks) that found both reduced anxiety and depressive feelings and improved sleep quality compared with placebo. Specifically, participants fell asleep faster, experienced fewer nocturnal disturbances, and reached for sleep medication less often. A caveat: the study is small, and two of the five authors are employed by the supplement manufacturer, which makes the results less independent¹.

Somewhat more research is available on the stress effect. A systematic review of nine randomised studies (200-400 mg per day) concludes that L-theanine can reduce stress and anxiety in people who are under stressful conditions. However, the reviewers emphasise that the evidence is not yet strong enough to recommend L-theanine as a treatment; larger and longer-term studies are needed for that². A separate crossover study in 34 healthy adults found that a single dose of L-theanine reduced both perceived stress and salivary cortisol levels after a stressful cognitive task. Because an objective measure (salivary cortisol) was also used here, this result is somewhat more credible than pure self-reporting³.

At the same time, there is also a clearly negative result. In an RCT with 80 participants and a more realistic, intensive stressor (a virtual shooting exercise), 200 mg of L-theanine had no measurable effect on salivary stress markers, heart rate, or anxiety scores. This shows that the stress-reducing effect of L-theanine is not self-evident and may depend on the type of stress or the circumstances⁴.

One study that did find stress reduction used a combination supplement containing, alongside L-theanine, magnesium, B vitamins, and rhodiola⁵. Because multiple substances were administered simultaneously, it is impossible to say how much L-theanine contributed. Furthermore, three authors were employed by the manufacturer Sanofi. This study therefore says little about L-theanine on its own.

A critical review from 2025 concludes that the scientific evidence for the popular health claims surrounding L-theanine, including relaxation, better sleep, and sharper cognitive function, is not yet strong enough. The researchers advise caution regarding the use of high doses by the general population as long as large, well-controlled studies are lacking. The safety profile of L-theanine does appear favourable based on toxicological research⁶. Animal research into nicotine withdrawal deserves a separate mention: mouse experiments show an interesting effect, but this says nothing about humans yet⁷.