L-theanine

Last reviewed October 2026 · Based on 18 studies

Bottom line

L-theanine, an amino acid from tea leaves, gives small gains in self-rated sleep in reviews of mostly small trials. Device-measured sleep results are mixed. Trials reported few side effects, but pregnancy, medicine interactions and long-term safety are not established.

What it helps with

Helps you fall asleep and sleep better — Benefit, evidence grade B (Moderate). Meta-analysis of 18 RCTs (897 people) and a systematic review of 13 trials (550); small effects; downgraded once for risk of bias (open-label trials included).
Improves sleep measured by a device — Mixed results, evidence grade C (Limited). Few small trials with conflicting results: one shows better sleep efficiency but no change in latency, another less light sleep than placebo; downgraded for inconsistency and small size.
Calms stress and anxiety — Mixed results, evidence grade C (Limited). Small trials with mixed results; one trial in adults with generalised anxiety found no benefit over placebo; downgraded for inconsistency and small size.
Improves attention and focus — Benefit, evidence grade C (Limited). Meta-analysis of tea and theanine trials (15 RCTs, co-funded by tea companies) found small-to-moderate benefits mostly with caffeine; theanine alone showed a small reaction-time effect; downgraded for inconsistency and indirectness.

What it is

L-theanine is a non-protein amino acid found in the leaves of the tea plant Camellia sinensis [1]. It is taken as a dietary supplement; the reviewed trials used 50 to 900 mg a day [1].

What the research says

A 2025 meta-analysis of 18 randomised trials with 897 people found small improvements in self-rated time to fall asleep (standardised effect 0.15), daytime problems (0.33) and overall sleep quality (0.43) [2]. A 2026 systematic review of 13 standalone L-theanine trials (550 people) reported benefits on objective and self-reported measures, including sleep latency, maintenance, efficiency and sleep satisfaction [1].

Device results are less consistent. In a six-week trial of 98 boys with ADHD, 400 mg a day improved sleep efficiency on actigraphy but did not change sleep latency [3]. In a 28-day trial of 30 stressed adults, the L-theanine group had less light sleep than placebo on actigraphy [4].

In a trial of 46 adults with generalised anxiety, L-theanine at 450 to 900 mg a day did not beat placebo on insomnia severity, though participants reported more sleep satisfaction [5].

Doses used in studies

The reviewers suggest 200 to 450 mg a day [1]. Trials used 200 mg a day for four weeks in adults [6], 400 mg a day in children with ADHD and in stressed adults [3][4], and 450 to 900 mg a day in adults with generalised anxiety [5].

Safety and interactions

Side effects: the ADHD sleep trial at 400 mg a day reported no significant adverse events [3]. In a 28-day trial at the same dose, all adverse events resolved and laboratory values were not clinically relevant [4]. The 2026 review describes 200 to 450 mg a day as safe [1].

Upper safe limits, pregnancy and breastfeeding, and interactions with medicines were not covered by the studies read, so the safety of higher doses and of combining L-theanine with prescription drugs is not established [1]. People who are pregnant or breastfeeding, have kidney or liver disease, or take medicines should ask a doctor first.

Caveats and open questions

Many trials are small, with 16 to 98 participants [7][3][4]. Several trials combined L-theanine with other ingredients, so their results cannot be credited to L-theanine alone [7]. The reviewers call for more high-quality trials using objective measures, for studies of mechanisms, and for people with clinical insomnia [1]. One anxiety trial found no benefit over placebo [5].