L-theanine is an amino acid found most commonly in tea leaves and in small amounts in Bay Bolete mushrooms. Research indicates that L-theanine promotes relaxation without drowsiness. Many people take L-theanine to help ease stress and unwind. (http://apjcn.nhri.org.tw/server/APJCN/17%20Suppl%201/167.pdf)
Five randomized-controlled trials with a total of 104 participants found L-theanine reduced stress and anxiety in people who were experiencing stressful situations. Another study found that it increased relaxation without causing drowsiness and reduced resting heart rate.
Researchers in one study found that doses of 250 mg and 400 mg of L-theanine greatly improved sleep in animals and humans. Also, 200 mg of L-theanine was shown to reduce resting heart rate, pointing to its ability to promote relaxation.