L-theanine comes from green tea and has been shown to improve deep sleep and to also help people maintain calm alertness during the day. Green tea also is helpful as an immune stimulant and has many other benefits.
People have used theanine and green tea historically as a way to reduce stress while remaining alert and avoiding drowsiness. Laboratory and animal tests indicate L-theanine may promote natural resistance to tumors and microbial infections. A study published in "Bioscience, Biotechnology and Biochemistry" in April 2002, for instance, found that dietary powdered green tea and theanine administered to rats had beneficial effects against induced liver cancer.