The 2015 American Academy of Sleep Medicines (AASM) guidelines for management of different circadian rhythm sleep-wake disorders support the use of strategically-timed melatonin for certain scenarios based on limited evidence (All WEAK recommendations):
Delayed sleep-wake phase disorder (DSWPD) in adults with/without depression and adolescents with/without psychiatric conditions.
24-hour sleep-wake rhythm disorder (N24SW) in blind adults.
Irregular sleep-wake rhythm disorders (ISWRD) in adolescents with neurological disorders. [1]
The 2017 AASM guidelines for primary insomnia do not recommend melatonin due to lack of quality evidence. An internal meta-analysis performed on three clinical trials in patients age > 55 years observing use of melatonin 2 mg nightly for 3 weeks did not find clinical significance (standardized mean difference [SMD] 0.21; 95% confidence interval -0.36 to 0.77) for sleep quality. Heterogeneity of included studies affected the review of other clinical parameters. Based on the limited data, melatonin did not observe significant clinical benefits on sleep latency, total sleep time, wake after sleep onset, quality of sleep, and sleep efficiency. [2]
The 2017 European Sleep Research Society guidelines for sleep disorders does not recommend melatonin for treatment of insomnia based on evidence from meta-analyses that question the clinical benefit. While melatonin may reduce sleep-onset latency and quality, the effect was modest at best (i.e. reducing time to sleep by only a couple of minutes). [3]
Natural Medicines Database supports the use of melatonin (likely effective) for delayed sleep phase syndrome (DSPS) to reduce the time to fall asleep. The conclusion was based on a series of clinical trials supported by meta-analysis which showed melatonin dose from 0.3 to 5 mg daily for 4 weeks improved sleep and quality of life. Long-term benefits, however, are not clear and there may be risk of relapse to pre-treatment sleeping status. Evidence for insomnia is also conflicting as the benefits seen were mostly a modest improvement in time to sleep with minor effect on sleep quality. [4]
Excretion of melatonin peaks between one to three years of age and is similar among genders. Endogenous melatonin secretion may decline with age. [5] People who suffer from an insufficient amount of environmental light may have decreased endogenous melatonin secretion. [6]
Melatonin supplementation is most efficacious when endogenous melatonin levels are low (e.g. during daytime or in individuals who produce insufficient amounts of melatonin). [7] Some conditions such as insomnia or depression can result in low levels of melatonin. [8]
A meta-analysis assessed fifty articles regarding melatonin safety, of those twenty-six found no statistically significant adverse events, while twenty-four articles reported on at least one statistically significant adverse event. The authors concluded melatonin adverse events were generally minor, short-lived, and easily managed, with the most commonly reported adverse events relating to fatigue, mood, or psychomotor and neurocognitive performance. [9]
Several studies, published more than twenty years ago, report exogenous melatonin does not suppress its endogenous production. [10], [11], [12]