«Melatonin is a hormone, produced by the pineal gland among other locations, which regulates wakefulness. As a medicine, it is used to treat insomnia; however, evidence of benefit is unclear. One review found onset of sleep occurred 6 minutes faster with use but found no change in total time asleep.
Side effects from supplements are minimal at low doses. In animals, melatonin is involved in the entrainment (synchronization) of the circadian rhythm including sleep-wake timing, blood pressure regulation, seasonal reproduction. Many of its biological effects are produced through activation of melatonin receptors, while others are due to its role as an antioxidant, particularly in the protection of nuclear and mitochondrial DNA. In plants it functions as a first line of defense against oxidative stress.
Melatonin was discovered in 1958. Melatonin is sold over the counter in the United States, Canada and some European countries. In other countries, it may require a prescription or be unavailable. A month's supply costs about 1 USD as of 2017.
Positions on the benefits of melatonin for insomnia are mixed. An AHRQ review from 2015 stated that evidence of benefit in the general population was unclear. A review from 2017 found a modest effect on time until onset of sleep. Another review from 2017 put this decrease at 6 minutes to sleep onset but found no difference in total sleep time. Melatonin may also be useful in delayed sleep phase syndrome. Melatonin appears to work as well as ramelteon but costs less.
Melatonin is a safer alternative than clonazepam in the treatment of REM sleep behavior disorder—a condition associated with the synucleinopathies like Parkinson's disease and dementia with Lewy bodies. In Europe it is used for short-term treatment of insomnia in people who are 55 years old or older. It is deemed to be a first line agent in this group.
Melatonin reduces the time until onset of sleep and increases sleep duration in children with neurodevelopmental disorders.
A 2016 review found no evidence that melatonin helped sleep problems in people with moderate to severe dementia due to Alzheimer's disease.
Melatonin is known to aid in reducing the effects of jet lag, especially in eastward travel, by promoting the necessary reset of the body's sleep-wake phase. If the timing is not correct, however, it can instead delay adaption.
Melatonin appears to have limited use against the sleep problems of people who work rotating or night shifts. Tentative evidence suggests that it increases the length of time people are able to sleep.
Melatonin appears to cause very few side effects as tested in the short term, up to three months, at low doses. Two systematic reviews found no adverse effects of exogenous melatonin in several clinical trials and comparative trials found the adverse effects headache, dizziness, nausea, and drowsiness were reported about equally for both melatonin and placebo. Prolonged-release melatonin is safe with long-term use of up to 12 months. Although not recommended for long term use beyond this, low-dose melatonin is generally safer, and a better alternative, than many prescription and over the counter sleep aids if a sleeping medication must be used for an extended period of time. Low-doses of melatonin are usually sufficient to produce a hypnotic effect in most people. Higher doses do not appear to result in a stronger effect, but instead appear to cause drowsiness for a longer period of time.
Melatonin can cause nausea, next-day grogginess, and irritability. In the elderly, it can cause reduced blood flow and hypothermia.[needs update] In autoimmune disorder, evidence is conflicting whether melatonin supplementation may ameliorate or exacerbate symptoms due to immunomodulation.[needs update]
Melatonin can lower follicle-stimulating hormone levels. Melatonin's effects on human reproduction remain unclear.» (wikipedia)