CRD Summary: This review assessed melatonin for adults with delayed sleep phase syndrome (DSPS) or primary insomnia (PI). The authors concluded that there is limited support for its use in people with DSPS, and little evidence to support its use for PI. These conclusions reflect the included evidence, which was poorly reported, but it is not clear whether they are reliable.

CRD Commentary: The review question and inclusion criteria were clear. The authors searched several databases and other relevant sources. However, they did not report a systematic search for unpublished studies, which might have increased the possibility that some relevant studies were not included in the review. The authors reported using methods designed to reduce bias and error in the assessment of study validity, but not in the study selection or data extraction processes. The validity assessment appeared to use appropriate criteria. The included studies were poorly reported and, in view of this, the authors' decision to adopt a narrative synthesis instead of a planned meta-analysis appears appropriate. The authors' cautious conclusions are an accurate reflection of the included evidence. However, the nature of this evidence, together with poor reporting of some aspects of the review methodology, mean it is not possible to be sure of their reliability.