Lemon Balm, Melissa officinalis is a perennial herb in the mint family, that is native to Europe, but has become naturalized in the United States. Like all mints, it has a square stem with opposite leaves, and fragrant leaves that give it its name. It grows from a few inches to a foot or two tall, and prefers moist soils, but in general, is a pretty hardy plant. It has many traditional medical uses, and their is a fair amount of research to back up at least some of the traditional uses of Lemon Balm.
I myself have used lemon balm to promote sleep and relaxation, and have found it helpful in that regard. If nothing else, it has a pleasant taste, and is without side-effects.