Melatonin improves sleep quality, but not for everyone

Melatonin is one of the most widely used sleep supplements in the world, yet its reputation runs ahead of the evidence in many settings. Individual trials have tested it in very different groups, from people with sleep disorders to those with respiratory or metabolic disease, and results have been mixed. Sleep quality, as opposed to simply falling asleep faster, is what most poor sleepers actually care about. This review pooled the randomised trials that measured sleep quality with a single validated questionnaire to see where melatonin genuinely helps.

The authors searched MEDLINE and EMBASE up to June 2020 for randomised clinical trials of melatonin in adults where sleep quality was assessed with the Pittsburgh Sleep Quality Index. From 2642 papers screened, 23 trials met the inclusion criteria. Results were pooled as mean differences using a random effects model. Because the trials covered people with a range of diseases, subgroup analyses were run by health status and by type of intervention.

Overall, melatonin improved sleep quality, lowering the Pittsburgh score by 1.24 points compared with control, where lower scores mean better sleep. The trials disagreed with each other substantially, so the subgroups mattered. Benefits were significant in people with respiratory diseases, in those with metabolic disorders, and in those with primary sleep disorders. No significant effect emerged in people with mental disorders, neurodegenerative diseases or other conditions.

The review supports melatonin as more than placebo for sleep quality, but with a clearly uneven map of who benefits. The strongest effects appeared in respiratory and metabolic disease, groups not usually front of mind when people reach for melatonin, while the effect in primary sleep disorders was smaller. The null results in mental and neurodegenerative conditions complicate any blanket claim that melatonin fixes sleep. Doses, formulations and durations varied across trials, leaving the optimal regimen unresolved.

This is a reason to think melatonin can genuinely improve overall sleep quality, not just shift sleep timing, at least in the populations studied. It is equally a reason for tempered expectations, since the trials were in adults with existing conditions and the benefit did not extend to every group. A healthy person with occasional bad nights was not really who these trials tested. The average improvement was modest, at 1.24 points on a questionnaire covering multiple aspects of sleep.