L-theanine

Regulatory status: dietary supplement ingredient; not an FDA-approved anxiety or insomnia treatment

Mechanism evidence: human

• Less situational / laboratory stress (subjective) after a single 200 mg dose.
• Lower physiological stress markers (HR, s-IgA, salivary α-amylase, salivary cortisol) after acute dosing.
• Lower trait anxiety or GAD symptom scores with repeated dosing.
• Better subjective sleep quality (PSQI / ISI / “slept well”).
• Faster sleep onset / shorter latency.
• Higher sleep efficiency / less wake after sleep onset (objective).
• Less daytime dysfunction / morning refreshment from better sleep.
• Treatment of clinical insomnia.
• Antidepressant effect (HAMD / SDS) as a primary claim.
• “Relaxed alertness” via more alpha activity (sold as the reason it works).

**Track A — isolated theanine for sleep / anxiety. Confidence: weak for clinical GAD and clinical insomnia; moderate for a small, short-term reduction in laboratory or everyday situational stress at 200 mg; weak-to-moderate for a small improvement in subjective sleep quality at 200 mg nightly.** Human RCTs at the typical **200 mg** dose show less HR/s-IgA reactivity (n=12), less clerkship stress (n=20, single-blind), and, after 4 weeks nightly, a within-person PSQI drop of **1.37 points** and STAI-trait drop of **3.37 points** that only partly survive a placebo comparison (Hidese n=30, Taiyo-funded). The 2025 sleep MA (SOL SMD 0.15; quality SMD 0.43) is consistent with a **small subjective** effect and is contaminated by combination products. The only GAD RCT (450–900 mg, n=46) is **null** on HAMA. There is **no** adult PSG insomnia trial of isolated theanine. Alpha-wave changes are real in high-trait-anxiety rest recordings and are not a sleep outcome. **What would change this:** an independent, pre-registered, 8–12 week RCT of 200 mg vs 400 mg vs placebo in DSM insomnia with PSG **and** PSQI, and a second GAD/anxiety RCT that actually moves HAMA.