Attention depends on coordinated brain signaling as well as the biological systems that sustain brain function. In ADHD, differences in dopamine and norepinephrine signaling are closely associated with attention, motivation, executive function, impulse control, and cognitive regulation. These processes are also shaped by sleep, stress physiology, nutritional status, metabolic function, and the cellular processes that support neuronal signaling and energy metabolism.

L-theanine is a naturally occurring amino acid found primarily in tea leaves that has been studied for its effects on attention, cognitive performance, stress, and sleep. Rather than acting through a single pathway, research suggests that L-theanine may modulate several processes involved in brain function, including glutamatergic signaling, stress regulation, and oxidative balance.

This broader range of biological effects has contributed to its growing interest in L-theanine within nutritional psychiatry and cognitive health research.

A 2026 randomized, double-blind, placebo-controlled crossover study published in Nutritional Neuroscience examined an L-theanine-caffeine combination in adolescents with ADHD. The combination reduced false alarms and attenuated the slowing of reaction time that occurred as the attention task continued, suggesting effects on response control and the stability of performance.

These findings build on earlier ADHD research and add to a broader body of evidence exploring L-theanine in relation to attention, cognitive performance, stress, and sleep.

New Evidence in ADHD & L-Theanine

The 2026 study included 21 adolescents between 10 and 19 years of age. Participants completed three conditions on separate days: L-theanine plus caffeine, placebo, and methylphenidate. Researchers evaluated selective attention using a computerized visual recognition task alongside EEG measures. The study used a counterbalanced, three-way crossover design, with testing performed before and 50 minutes after each treatment.

The most notable behavioral finding was a reduction in false alarms following the L-theanine-caffeine combination compared with placebo. Participants were less likely to respond to irrelevant stimuli. The combination also attenuated the progressive slowing of reaction time that occurred as the task continued.

Importantly, the combination did not significantly improve overall reaction time compared with placebo; methylphenidate did. Rather, L-theanine-caffeine appeared to counteract the decline in reaction speed that developed as the cognitive task progressed.

This distinction is important, as selective attention involves more than responding quickly. It requires the ability to identify relevant information, suppress competing signals, and sustain accurate performance as cognitive demands increase

The researchers also observed changes in brain activity associated with attentional processing following both the L-theanine-caffeine combination and methylphenidate. P3b is an EEG marker associated with attentional processing and the allocation of cognitive resources. Both treatments increased P3b amplitude and decreased P3b latency compared with placebo, reflecting changes in this marker of attentional processing.

Together, the behavioral and EEG findings suggest effects on specific components of attentional processing and response control.

The study provides an early clinical signal regarding how L-theanine, particularly in combination with caffeine, may influence attention and response control in adolescents with ADHD. However, it evaluated an acute, high-dose combination in a small sample, so the findings should be considered preliminary.

Notably, the 2026 trial evaluated L-theanine with caffeine, meaning its findings cannot be attributed to L-theanine independently.

Earlier ADHD Research: Building on the Evidence

The 2026 findings build on a much smaller 2020 randomized neuroimaging study published in Scientific Reports. Five boys with ADHD received L-theanine, caffeine, the combination, or placebo. Researchers assessed sustained attention, inhibitory control, overall cognition, and brain activity using cognitive testing and fMRI.

L-theanine alone improved the overall cognition composite compared with placebo. The L-theanine-caffeine combination also improved overall cognition and sensitivity to relevant signals on a Go/NoGo task, which requires participants to respond to target signals while withholding responses to non-target signals. The combination showed a trend toward improved inhibitory control.

The neuroimaging findings offered a complementary view of these effects. During the Go/NoGo task, the L-theanine-caffeine combination was associated with reduced task-related reactivity of the default mode network, a brain network associated with internally directed thought and mind wandering. During the Stop-signal task, the combination was associated with changes in activity within the default mode and central executive networks.

Although the findings are intriguing, the study included only five participants and was designed as a proof-of-concept trial. The results therefore provide preliminary evidence rather than establishing a clinical effect of L-theanine in ADHD.

What Broader Research Reveals

The ADHD-specific findings also fit within a broader body of research exploring L-theanine and cognitive performance in healthy and clinical populations.

A 2026 systematic review and meta-analysis published in Molecular Psychiatry examined 31 randomized controlled trials involving 1,168 participants. The review evaluated L-theanine in relation to cognitive performance, stress, mood, fatigue, and other outcomes. A single 200-mg dose taken 30-60 minutes prior to cognitive testing significantly improved choice reaction time in healthy adults.The review also found a modest reduction in acute stress.

These findings provide broader context for the emerging ADHD research, while the populations and outcomes differ from those of the ADHD-specific trials.

L-Theanine, Sleep & Stress

Attention is closely connected to sleep and stress physiology. Both influence the biological conditions that allow the brain to sustain alertness, regulate competing signals, and maintain cognitive performance.

Sleep influences brain metabolism, hormone and immune function, and the cellular processes that support recovery, while also shaping daytime alertness and executive function. Sleep difficulties are common in ADHD and can further affect cognitive performance.

A systematic review published in 2026 in Nutritional Neuroscience examined 13 human trials involving 550 participants and evaluated L-theanine as a standalone intervention for sleep. The included studies used doses ranging from 50 to 900 mg/day, although 200–450 mg/day was used in all but two trials. Nine of 13 trials reported statistically significant or trend-level improvements in at least one sleep-related measure, including sleep latency, sleep maintenance, sleep efficiency, perceived sleep quality, or morning refreshment.

The trials varied considerably in design, dose, and duration. The most consistent findings involved measures of sleep quality, including sleep efficiency and perceived sleep quality. Evidence that L-theanine increases total sleep time was limited

Because the sleep review was conducted in adults and was not designed specifically to evaluate ADHD symptoms, these findings do not establish an effect on ADHD. They do, however, provide relevant context for understanding sleep as one of the physiological factors that can influence attention and cognitive function.

Stress physiology provides another important connection.The hypothalamic-pituitary-adrenal (HPA) axis and autonomic nervous system (ANS) coordinate the body’s response to challenge. Persistent stress can influence cortisol regulation, sympathetic activity, sleep, metabolic function, immune signaling, and emotional regulation. Sleep disruption can, in turn, make stress regulation more difficult, creating a physiological cycle that may affect daytime alertness and cognitive performance.

Together, the sleep and stress literature provides additional context for understanding the physiological conditions that may influence attention and cognitive performance in ADHD.

L-Theanine in the Broader Picture of ADHD

The emerging research on L-theanine is particularly interesting because it intersects with several dimensions of ADHD, including attention, response control, stress, and sleep.

The 2026 ADHD trial found fewer false alarms, more stable reaction-time performance, and changes in brain activity associated with attentional processing following an L-theanine-caffeine combination. The earlier 2020 study provided preliminary evidence of cognitive effects with both L-theanine alone and the combination. The 2026 Molecular Psychiatry meta-analysis adds broader evidence for effects on cognitive performance and stress, while the sleep review identifies potential benefits across several measures of sleep quality.

Taken together, these findings place L-theanine within an emerging area of nutritional psychiatry research, although the evidence remains preliminary.

The strongest ADHD-specific evidence currently involves L-theanine in combination with caffeine, while research on L-theanine alone provides broader context for its potential effects on cognition and sleep.

For clinicians taking an integrative approach to ADHD, L-theanine is best viewed as an emerging area of research. Its potential relevance can be considered alongside the individual biological factors that influence attention, including sleep, stress, circadian rhythm, nutritional adequacy, and metabolic health.

This broader perspective is consistent with Dr. James Greenblatt’s multifaceted approach to ADHD, explored in Finally Focused: The Breakthrough Natural Treatment Plan for ADHD, which considers nutritional status, sleep, gut health, and other biological factors alongside conventional treatment.

References

• Nawarathna, G. S., Ariyasinghe, D. I., Balasooriya, N., Fernando, A., & Dassanayake, T. L. (2026). Effects of L-theanine-caffeine combination on selective attention among adolescents with attention-deficit hyperactivity disorder: a double-blind, placebo-controlled, crossover study. Nutritional Neuroscience, 1–14. https://doi.org/10.1080/1028415X.2026.2659148
• Kahathuduwa, C. N., Wakefield, S., West, B. D. et al. (2020). Effects of L-theanine–caffeine combination on sustained attention and inhibitory control among children with ADHD: a proof-of-concept neuroimaging RCT. Scientific Reports, 10, 13072. https://doi.org/10.1038/s41598-020-70037-7
• Gerolymos, C., Saddier, E., Boyer, L. et al. (2026). Cognitive and affective effects of L-theanine: a systematic review and meta-analysis of 31 randomized trials. Molecular Psychiatry. https://doi.org/10.1038/s41380-026-03727-9
• Cotter, J., Caddick, C. E., Harper, J. L., & Ebajemito, J. K. (2026). Examining the effect of L-theanine on sleep: a systematic review of dietary supplementation trials. Nutritional Neuroscience, 29(2), 224–238. https://doi.org/10.1080/1028415X.2025.2556925