L-theanine is a unique type of amino acid that is normally not found in our diet, nor is it produced by our body. In many regards it is very similar to L-Ornithine and L-Citrulline.
When used it promoted a relaxed state of mind, but it doesn't sedate the body. It also reduces the perception of stress and produces beneficial effects for attention and cognition. L-theanine doesn't promote sleep, but it could potentially help with various sleep disorders. There are many different treatments for these situations, with a much better track record.
What is unique about L-theanine is it's synergistic effects with other stimulating nootropics. Combinating this amino acid with another high energy substance can yield many additional benefits in the form of increased attention, ease of focus and overall brain performance.