There is a review of the efficacy of herbal remedies in treating insomnia. The aim of the study was to review the published literature on treating insomnia with herbal remedies.
Valerian root was shown to probably be safe, but the studies regarding efficacy were inconclusive. Kava-kava was shown in one study to be effective, but safety concerns including skin reactions, neurological complications, and liver damage have prompted the FDA to issue an advisory statement about its use. Chamomile and St. John's wort have very few articles supporting their use in treating insomnia. Lemon balm, passionflower, and lavender have no published articles on their efficacy in insomnia.
While the temptation to try these herbal remedies is understandable, just because a compound seems "natural" does not mean it is safe and/or effective in managing insomnia. Cognitive behavioral therapy (CBT) has been shown to be safe and effective for insomnia, and does not involve ingesting any substances. With CBT, you learn skills that will help you sleep for the rest of your life.