BACKGROUND: The aim of this study was to determine the efficacy of exogenous melatonin supplementation for sleep disturbances in patients with middle-aged primary insomnia. METHODS: This is a randomized double-blind, placebo-controlled parallel study. Participants were recruited from Tianlin community, Xuhui district, Shanghai. Ninety-seven consecutive middle-aged patients with primary insomnia were randomized to receive 3 mg fast-release melatonin (n = 51) or placebo (n = 46) for four-weeks. Objective sleep parameters tested by overnight polysomnography, subjective sleep performance and daytime somnolence obtained from the Pittsburgh Sleep Quality Index (PSQI), Insomnia Severity Index (ISI) and Epworth Sleepiness Scale (ESS) were obtained at baseline and after treatment. Treatment was taken daily 1 h before bedtime. Serious adverse events and side-effects were monitored. RESULTS: Melatonin supplementation significantly decreased early wake time [-30.63min (95% CI, -53.92 to -7.34); P = 0.001] and percentage of N2 sleep [-7.07% (95% CI, -13.47% to -0.68%); P = 0.031]. However, melatonin had no significant effect on other objective sleep parameters including sleep latency, sleep efficiency, wake during the sleep and percent of N1, N3 and REM sleep. Melatonin had no effect on insomnia symptoms and severity on the PSQI [1.53(95% CI, -0.55 to 3.61); p = 0.504]; ISI [0.81 (95% CI, -2.27 to 3.88); p = 0.165] and ESS [-0.83 (95% CI, -3.53 to 1.88); p = 0.147]. No serious adverse events were reported. CONCLUSIONS: Melatonin supplementation over a four-week period is effective and safe in improving some aspects of objective sleep quality such as total sleep time, percentage of rapid eye movement and early morning wake time in middle-aged patients with insomnia. TRIAL REGISTRATION: Identifier: ChiCTR-TRC-13003997; Prospectively registered on 2 December 2013.