L-theanine: stress regulation, sleep quality, and neurocognitive resilience
Long-term chronic mental strain affects both mental and physical health. It disrupts sleep, raises signals of inflammation, and overburdens the nervous system—processes that contribute to inflammaging and disrupted communication between cells, two important Hallmarks of Aging. L-theanine is a naturally occurring amino acid found mainly in green and black tea that can shape these strain responses on several levels.
At the level of nerve function, L-theanine influences the activity of key messenger substances in the brain, including γ-aminobutyric acid (GABA), dopamine, and glutamate, and it is linked with an increase in alpha brain waves that are typical of a relaxed, awake state without drowsiness.¹ ⁵ Studies using electroencephalography (EEG) show that L-theanine may help shift brain-wave patterns toward a state of focused relaxation, which can soften strain signals and support mental clarity.
Human clinical studies, including randomized controlled trials, show that supplementation with L-theanine can contribute to reduced self-reported strain and strain-related symptoms, improved sleep quality, and improved mental performance in healthy adults.² In an RCT in healthy adults, L-theanine was linked with lower levels of strain-related complaints, and other research shows improvements in attention and working memory after supplementation.⁴
Systematic reviews and meta-analyses of sleep studies suggest that L-theanine can support sleep quality, with improvements in self-reported sleep measures such as sleep quality and sleep onset latency (how quickly someone falls asleep) in adults using L-theanine. Although objective measurements (such as total sleep time) differ between studies, the overall trend points to beneficial effects on perceived sleep quality and daytime functioning.
In terms of how it works, L-theanine supports cognitive flexibility and emotional steadiness by tempering strain responses. This likely stems from shaping messenger substances in the brain and balancing sympathetic activation during mental load, which may help maintain focus, mood, and recovery during periods of strain.²
Importantly, L-theanine is not a sedative; it does not work by simply suppressing the central nervous system, but rather by helping it return to a balanced, resilient state. For this reason, the supplement can be part of a broader long-life strategy aimed at supporting the stress axis, inflammaging, and neural integrity without reducing functional alertness.
Sources:
- Dashwood R. L-theanine and brain function: from mechanism to benefit. SciDirect 2025, review of how L-theanine may modulate brain waves, stress and relaxation.
https://www.sciencedirect.com/science/article/pii/S0271531724001684 - Hidese S, et al. Effects of L-Theanine Administration on Stress-Related Symptoms and Cognitive Functions in Healthy Adults. PMC Free Article 2019, RCT showing stress and cognitive benefits with L-theanine.
https://pmc.ncbi.nlm.nih.gov/articles/PMC6836118/ - Bulman A, et al. The effects of L-theanine consumption on sleep outcomes. Systematic analysis 2025, finds L-theanine supports subjective sleep quality and sleep onset.
https://www.sciencedirect.com/science/article/pii/S1087079225000292 - Baba Y, et al. Effects of L-theanine on Cognitive Function in Middle-Aged Adults. Clinical research showing improvements in attention and working memory.
https://pmc.ncbi.nlm.nih.gov/articles/PMC8080935/
