Objective: Sleep problems are a common complaint among adults worldwide, however, the use of prescription and over-the-counter products may not always be an appropriate or desirable solution. L-theanine is a naturally occurring, non-protein amino acid that can be found in the leaves of the tea plant Camellia sinensis. Previous studies have reported that consumption of L-theanine can help to aid relaxation, without causing sedation or adversely impacting cognitive function. Building on these calming effects and results from recent pre-clinical studies, the aim of this review was to systematically appraise the scientific literature to establish whether dietary supplementation with L-theanine can also help to support sleep in humans.Methods: Electronic database searches of Ovid MEDLINE, PsycINFO, Embase, CENTRAL and Google Scholar were conducted from inception to 3rd February 2025. Retrieved articles were independently reviewed by three authors.Results: Thirteen eligible trials (n = 550) that examined the effect of L-theanine (50-900 mg/day) as a standalone intervention on sleep-related outcome measures were identified. This included two single-arm, open-label trials and eleven randomised controlled trials.Discussion: Based on the current evidence, supplementation with 200-450 mg/day of L-theanine appears to be a safe and effective way to support healthy sleep in adults. Among the included trials, beneficial effects were reported on both objective and participant-reported outcomes, including measures linked to sleep latency, maintenance and efficiency, perceived sleep satisfaction and feelings of refreshment and recovery on waking. Further high-quality trials using objective measures, into the mechanisms underlying these effects, and among those with clinical insomnia would provide further useful insights.
Keywords: L-theanine; review; sleep; supplement; trial.