Oral melatonin supplementation has been heralded as a treatment for the sleep disorders of delayed sleep phase syndrome (DSPS) and primary insomnia (PI) in adults (18 to 65 years). However, there is inconsistency in the literature and many of the claims for melatonin appear to be based upon its effect in older adults ( > 65 years). This article reviews, systematically, studies of oral melatonin in adults (18 to 65 years) with either delayed sleep phase syndrome (DSPS) or primary insomnia (PI). Following electronic database searching, and hand-searching of relevant journal titles, fourteen articles that examined the use of melatonin in adults with PI or DSPS were identified for inclusion in this review (eight for DSPS and four for PI). Generally, the quality of articles is limited, particularly with regard to reporting of sleep parameters. However, results indicate that melatonin may phase-advance the sleep of individuals diagnosed with DSPS, although the majority of this evidence comes from uncontrolled trials. There is little evidence to suggest that melatonin is an effective treatment for PI. Further research, addressing deficiencies in the current literature, and considering long-term efficacy and safety are necessary before definitive conclusions as to the value of oral melatonin can be drawn.