Valerian (Valeriana officinalis) is a plant with a long history of use as a medicinal plant for a large number of disorders. It is said to have been used to treat insomnia since pre-Christian times. The Peterson Field Guide to Medicinal Plants states that it is the leading over-the-counter tranquilizer in Europe. And cats like it, too! But, for cats, it acts as a stimulant rather than as a sedative. The roots of valerian contain actinidine, and it is likely this substance which makes it attractive to cats.