The purpose of this study was to investigate the efficacy and safety of L-theanine as an aid to the improvement of objectively measured sleep quality in a population of 98 male children formally diagnosed with attention-deficit/hyperactivity disorder (ADHD). METHODS: A randomized, double-blind, placebo-controlled trial was conducted involving boys, ages 8-12 years, who had been previously diagnosed with ADHD. An experienced physician confirmed the diagnosis of ADHD in each subject. Randomization was stratified based upon current use of stimulant medication to ensure an equal distribution of stimulant/non-stimulant treated subjects into active and placebo treated groups. Participants consumed two chewable tablets twice daily (at breakfast and after school), with each tablet containing 100 mg of L-theanine (total 400 mg daily Suntheanine, Taiyo Kagaku, Yokkaichi, Japan) or identical tasting chewable placebo for six weeks. Subjects were evaluated for five consecutive nights using wrist actigraphy at baseline, and again at the end of the six-week treatment period. The Pediatric Sleep Questionnaire (PSQ) was completed by parents at baseline and at the end of the treatment period. RESULTS: Actigraph watch data findings indicated that boys who consumed L-theanine obtained significantly higher sleep percentage and sleep efficiency scores, along with a non-significant trend for less activity during sleep (defined as less time awake after sleep onset) compared to those in the placebo group. Sleep latency and other sleep parameters were unchanged. The PSQ data did not correlate significantly to the objective data gathered from actigraphy, suggesting that parents were not particularly aware of their children’s sleep quality. L-theanine at relatively high doses was well tolerated with no significant adverse events. CONCLUSIONS: This study demonstrates that 400 mg daily of L-theanine is safe and effective in improving some aspects of sleep quality in boys diagnosed with ADHD. Since sleep problems are a common comorbidity associated with ADHD, and because disturbed sleep may be linked etiologically to this disorder, L-theanine may represent a safe and important adjunctive therapy in childhood ADHD. Larger, long-term studies looking at the wider therapeutic role of this agent in this population are warranted.