How long does the effect of melatonin last? The general effect of melatonin lasts mainly 6-8 hours, with its peak effect time being about 4 hours. Melatonin is a major hormone secreted by the pineal gland in our body, usually at night, mainly because it promotes sleep. There is one more, its main use is due to the reversal of circadian rhythms, especially long-distance flights lead to disruption of melatonin secretion, when it is necessary to use melatonin. Melatonin is not effective for all people, some patients are not effective with melatonin. Who should use melatonin? Always go to the neurology department of a regular hospital and let the doctor determine if the patient should use melatonin.