We conducted the first systematic review and series of meta-analyses to assess the efficacy and tolerability of melatonin in children/adolescents or adults with sleep or mental health disorders, using the same set of criteria across disorders and ages. Based on a pre-registered protocol (PROPSPERO: CRD42021289827), we searched a broad range of electronic databases up to 02.02.2021 for randomized control trials (RCTs) of melatonin. We assessed study quality using the Risk of Bias tool, v2. We…