Abstract: L-theanine is one kind of unique non-peptide amino acid of tea, which has a variety of physiological functions, such as protecting nerves, lowering blood pressure, anti-oxidative activity, regulating immune response, cognitive performance improvement and other physiological functions. Because of its tranquilizing and tranquilizing properties, the research of L-theanine in improving sleep increases gradually. In this paper, the metabolism and the mechanism of L-theanine in improving sleep were reviewed. This aims to provide a reference for the further research in the sleep improvement and the further exploitation and utilization of L-theanine.