Two umbrella reviews, 7 systematic reviews, and 2 randomized controlled trials provided mixed results on the clinical effectiveness of melatonin for insomnia, when compared to placebo. Some studies reported improvement in sleep and quality of life outcomes with melatonin, and some studies reported no difference between patients who received melatonin and those who received placebo. Efficacy of melatonin was measured both objectively (e.g., polysomnography, actigraphy) and subjectively (e.g., validated questionnaires, sleep diaries), and was measured across multiple outcomes. Two guidelines recommend melatonin for insomnia, but the strength of the recommendations was not reported. One guideline recommends melatonin for insomnia, based on very low evidence (but the evidence was unclear). One guideline recommends against melatonin for chronic insomnia disorder (weak recommendation). The evidence for these recommendations was not well reported across the guidelines. No studies were found that evaluated the clinical effectiveness of melatonin compared to prescription sedatives in people with insomnia that met the criteria for this review. No studies were found for the cost-effectiveness of melatonin in people with insomnia that met the criteria for this review.