L-Theanine
Amino AcidAlso known as: Theanine · L-γ-glutamylethylamide · 5-N-Ethyl-glutamine
L-Theanine is an amino acid found naturally in green and black tea. It is studied for its ability to promote relaxed alertness, reduce anxiety and stress responses, improve sleep quality, and enhance the cognitive effects of caffeine. Unlike sedatives, it does not cause drowsiness at typical doses.
Evidence Summary
All 5 studiesL-Theanine is an amino acid found naturally in green tea (Camellia sinensis) that has been studied for its effects on cognitive performance, anxiety, stress, and sleep. It is thought to act primarily by increasing alpha-band neural oscillations, modulating glutamate and GABA pathways, and influencing serotonin and dopamine signaling. The most consistent evidence concerns L-theanine's combination with caffeine for cognitive performance. Multiple RCTs demonstrate that the combination (commonly 100–200 mg L-theanine with 50–100 mg caffeine) improves sustained attention, working memory accuracy, and reaction time compared to caffeine alone, while attenuating some of caffeine's stimulant side effects. This combination is well-supported by both performance data and EEG evidence of altered brain state. For anxiety and stress, evidence is encouraging but limited by small trial sizes. Several RCTs (n=16–44) found L-theanine (200 mg, acute or short-term) reduced subjective anxiety, stress biomarkers, and heart rate response to stressors. A 4-week crossover trial (Hidese et al., 2019, n=30) found improvements in stress-related symptoms including sleep disturbance, depression-like symptoms, and cognitive function. However, all trials are small and conducted in healthy adults without clinical anxiety disorders — the evidence does not establish efficacy for anxiety disorders. Evidence for sleep is preliminary. Alpha-wave promotion and GABA modulation provide a plausible mechanism for sleep benefits, and the Hidese et al. (2019) study noted improvements in sleep disturbance measures. Dedicated sleep endpoint RCTs are limited. Safety data is favorable. L-theanine is Generally Recognized as Safe (GRAS) by the FDA. No significant adverse effects have been reported in trials up to 400 mg/day. Long-term safety data beyond 12 weeks is limited in the published literature.
Read full evidence summary →Top studies
L-theanine and caffeine in combination affect human cognition as evidenced by oscillatory alpha-band activity and attention task performance
L-theanine alone and in combination with caffeine increased alpha-band activity, suggesting a relaxed but alert mental state. Combined L-theanine + caffeine improved attention task speed without reducing accuracy.
L-theanine may promote alert relaxation; combination with caffeine may improve attention
None reported
Small sample, acute dosing only, healthy young adults
The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness
Combined L-theanine (97mg) and caffeine (40mg) improved accuracy on demanding cognitive tasks and increased alertness more than caffeine alone. L-theanine appeared to reduce some caffeine-associated jitteriness.
L-theanine + caffeine may improve sustained attention and cognitive accuracy, with theanine potentially reducing caffeine side effects
None beyond caffeine effects
Young healthy adults, acute dosing, industry funding
Expert Mentions
All 4 mentions"The combination of theanine and caffeine is something I find really useful. The theanine takes the edge off the caffeine — you get the alertness and focus without the jitteriness."
L-Theanine paired with caffeine takes the "edge" off caffeine and promotes a calm, focused mental state without sedation.
Multiple RCTs (Haskell et al., Foxe et al.) support the combination of L-theanine and caffeine for improved cognitive performance. L-theanine's attenuation of caffeine side effects (jitteriness, cardiovascular) has specific EEG and performance support.
"L-theanine, 100 to 200 milligrams, can be very effective for people who have trouble with a racing mind at night. It tends to quiet mental chatter without making you feel groggy."
L-Theanine at 100–200 mg before sleep can help quiet a racing mind and improve sleep onset.
Hidese et al. (2019) found L-theanine (200mg/day) improved sleep disturbance symptoms over 4 weeks. The specific effect on "racing mind" is plausible given alpha-wave promotion, but direct RCT evidence for sleep onset in otherwise healthy adults is limited.
Key findings
- ·Consistent RCT evidence that L-theanine + caffeine improves attention, accuracy, and reduces caffeine jitteriness.
- ·Several small RCTs suggest L-theanine (200 mg) reduces acute anxiety and stress biomarkers.
- ·EEG evidence confirms L-theanine increases alpha-band neural activity, associated with relaxed alertness.
Evidence gaps
- ·Most trials are small (n<50) and acute; long-term effects in clinical populations are understudied.
- ·No RCTs specifically in people with diagnosed anxiety disorders.
- ·Dedicated sleep RCTs (with polysomnography) are lacking.