L-theanine is an amino acid that impacts nerve impulses in the brain and the release of neurotransmitters. It can support overall mental health and energy levels and acts as a natural stress-reliever. It can help improve your ability to deal with stress without making you feel lethargic or tired. L-theanine and coffee are usually found in beverages together. L-theanine tends to promote relaxation, while caffeine promotes alertness, the two have opposite but complementary effects. Studies suggest that both L-theanine and caffeine can have beneficial effects on cognition and mood when used appropriately. Source