L-Theanine is a extract from Green Tea. Studies using electroencephalography have shown the ability of L-theanine to increase α-brainwave activity, which is indicative of a state of wakeful relaxation. Alpha activity has also been associated with increased creativity, increased performance under stress, and improved learning and concentration, as well as decreased anxiety so if L-theanine impacts alpha activity, then it may also be associated with these psychological outcomes.(Bryan J.,Psychological effects of dietary components of tea: caffeine and L-theanine. 2008 Feb)
It seems to settle excitable neurotransmitters while promoting calming neurotransmitters like serotonin, dopamine and GABA. L-Theanine has been suggested by researchers to modulate the brain's 'attention circuitry.' It may also protect neurons from damage, potentially helping to maintain clear thinking during the aging process.
Clinical tests on Boys aged between 8-15 yrs with ADHD in 2020 suggest the reversal of task-related mind-wandering and improved inhibitory control.(Kahathuduwa CN, Wakefield S, West BD, Blume J, Dassanayake TL, Weerasinghe VS, Mastergeorge A: Effects of L-theanine-caffeine combination on sustained attention and inhibitory control among children with ADHD: a proof-of-concept neuroimaging RCT.)