L-Theanine

L-theanine (γ-glutamylethylamide), a non-protein amino acid found almost exclusively in tea, is one of the better-studied nootropic/relaxation supplements, with dozens of randomized controlled trials and several systematic reviews and meta-analyses in humans. The most consistent, well-replicated findings are for acute effects: a single dose of roughly 100-250 mg promotes a state of calm alertness ('relaxation without sedation'), increases occipital/frontal alpha-band EEG and MEG activity, and modestly blunts acute stress-reactivity markers such as salivary cortisol. Combined with caffeine (commonly ~100 mg L-theanine + ~50 mg caffeine), it reliably improves attention, target discrimination and reaction time while reducing caffeine-related jitter. A 2026 meta-analysis of 31 RCTs (n=1,168) found a 200 mg dose significantly improved choice reaction time/attention and reduced depressive symptoms after single dosing, with a modest effect on acute stress but inconsistent, largely non-significant effects on anxiety. Evidence for sleep is positive but weaker: a 2025 Sleep Medicine Reviews meta-analysis (18 studies, N=897) found small-to-moderate improvements in subjective sleep quality, sleep-onset latency and daytime dysfunction, though most trials used combination products rather than pure L-theanine. Chronic supplementation (e.g., 200 mg/day for 4 weeks) has shown benefits for stress, sleep and executive function in one Nutrients RCT, but long-term cognitive benefit is not established. Typical studied doses are 100-400 mg/day; L-theanine is generally well tolerated with an excellent safety profile, and holds GRAS status in the US. Overall consensus is moderate: robust for acute relaxation/alpha activity, attention (especially with caffeine) and safety; emerging and mixed for anxiety, chronic effects and standalone sleep benefit.