Newer insomnia drugs include Ambien, Lunesta, and Sonata. These are similar, but chemically different than older sleep aids like Valium or Restoril. Here is an article that summarized 13 clinical trials of the newer insomnia drugs as submitted to the FDA. All of these trials were placebo-controlled. The trials involved sleep studies and subjective ratings by the trial participants.
The results showed significant reductions in time to fall asleep in both the drugs and the placebo, but the drugs did reduce time to fall asleep more than the placebo. However, the difference is small and clinical significance is not known. As documented on the sleep study, it took participants 22 minutes longer to fall asleep in the placebo group. As per subjective rating, participants felt it took them 7 minutes longer to fall asleep on placebo than on the drug.
The study authors concluded from their analysis that the placebo response is a major contributor to the effectiveness of these newer sleep medications. In other words, these new drugs work because participants think they are taking a medication for sleep, not necessarily because the medicine actually makes them sleep better.