A 2007 randomized, double blind, placebo-controlled, parallel- group, multi-center study of 170 primary insomnia subjects evaluated the effect of 2mg of melatonin on reducing sleep latency (time to sleep) and improving sleep quality. The melatonin treatment group significantly improved quality of sleep and morning alertness without withdrawal symptoms upon discontinuation.

A 2009 randomized, double-blind sleep laboratory phase 2 study of 40 subjects evaluated the effect of 2mg of melatonin on sleep measures and psychomotor performance. The study found that the treatment group had significantly shorter sleep onset latency compared with the placebo group and scored significantly better in the Critical Flicker Fusion Test. Half of the treatment subjects reported substantial improvement in sleep quality at home taking melatonin compared with 15% with placebo. Study researchers concluded that nightly treatment with melatonin effectively induced sleep.