This is commonly used as a remedy for insomnia. One placebo-controlled study showed that people taking valerian (400 mg of extract each night for 2 weeks) fell asleep quicker and had better sleep quality without next-day sedation. Another placebo-controlled study using 450 and 900 mg doses for just 1 night also showed improved sleep, but there was some hangover with the higher dose. A number of other studies also show benefit, including an improvement in deep sleep. The benefits were most pronounced when people used it for extended periods as opposed to simply taking it for 1 night. Another study showed it to be as effective as a Valium family medication (Oxazepam®). A review of multiple studies found that "valerian is a safe herbal choice for the treatment of mild insomnia and has good tolerance. Most studies suggest that it is more effective when used continuously rather than as an acute sleep aid."38
38Hadley, S., et al. “Valerian.” American Family Physician, 2003; 67 (8): 1755 – 1758.