L-theanine is an amino acid found in green and black tea. When given as a supplement in amounts higher than in a cup of tea, preliminary studies suggest a number of effects, such as improving mental alertness, reducing stress, and enhancing the quality of sleep -- although it does not cause drowsiness. has tested the quality of popular brands of L-theanine. You can find out how these L-theanine supplements fared in the tests; how they compare on quality and ingredients; which offer the best value; and learn about proper dosage and potential side effects in's Review of L-Theanine Supplements >>

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January 13, 2015

Hi Don - Thank you for adding these studies! You can read more about these in the Encyclopedia article about L-theanine:

January 12, 2015

Haskell CF, Kennedy DO, Milne AL, Wesnes KA, Scholey AB. The effects of L-theanine, caffeine, and their combination on cognition and mood. Biol Psychol . 2008;77(2):113-122.

January 12, 2015

Kimura K, Ozeki M, Juneja LR, Ohira H. L-Theanine reduces psychological and physiological stress responses. Biol Psychol . 2007;74(1):39-45

May 27, 2020

I have had good experience with l'theanine, which I found after reading research studies. I take it at bedtime. It helps makes my sleep more restful and alleviates some of the aches and pains that I would otherwise have from an unrestful night. My sister occasionally gives my niece a small dose of chewable before bedtime as needed.

March 14, 2018

If one has sleep disorder known as alpha intrusion (alpha waves interrupt deep sleep delta waves), would taking L-theanine make that worse? are there any studies on this?
March 15, 2018

Hi Anna - As noted in the L-Theanine Supplements Review, there is some evidence that L-theanine may have an effect on alpha brain waves (; however we are not aware of any research specifically on the effects of L-theanine supplementation in individuals with this sleep disorder.

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