Focused Relaxation. L-Theanine is a unique amino acid found almost exclusively in the tea plant (Camellia sinensis) and contributes to the unusual taste of green tea. Human studies have shown that taking L-theanine affects the emission of alpha waves in the brain associated with states of relaxation and focused attention. Preliminary research has shown that L-theanine crosses the blood-brain barrier, and suggests that it may support healthy neurotransmitter levels in the brain, and inhibit the stimulatory properties of caffeine