One of the best designed studies found that valerian was no more effective than placebo for 14 days, but by 28 days valerian greatly improved sleep for those who were taking it.
Valerian has been used to ease insomnia, anxiety, and nervous restlessness since the second century A.D. It became popular in Europe in the 17th century. It has also been suggested to treat stomach cramps. Some research -- though not all -- suggests that valerian may help people with insomnia. Germany's Commission E approved valerian as an effective mild sedative and the United States Food and Drug Administration listed valerian as "Generally Recognized As Safe" (GRAS).
Scientists aren't sure how valerian works, but they believe it increases the amount of a chemical called gamma aminobutyric acid (GABA) in the brain. GABA helps regulate nerve cells and has a calming effect on anxiety. Drugs such as alprazolam (Xanax) and diazepam (Valium) also work by increasing the amount of GABA in the brain. Researchers think valerian may have a similar, but weaker effect.
Full Article: https://www.mountsinai.org/health-library/herb/valerian