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Melatonin is a derivative of serotonin best known for its role as a hormone.
It is produced in a small endocrine gland in the brain called the pineal gland, as well
as in the retina and a number of other organs and cells.
The melatonin produced in the pineal gland, however, is thought to contribute most to
circulating levels of melatonin in the bloodstream.
Synthesis of melatonin in the pineal gland, and resultant levels of melatonin in the bloodstream,
exhibit a clear circadian rhythm, where they are highest at night.
This nighttime melatonin production is stimulated by neural input from a structure called the
suprachiasmatic nucleus, which acts as a master circadian clock for the brain.
Indeed, the best understood function of melatonin is its role in regulating circadian rhythms.
Rising melatonin levels at night are associated with lower body temperature and blood pressure,
and an increase in tiredness.
Melatonin production can be inhibited by exposure to blue wavelength light, which is part of
the basis for advice to avoid using electronic devices close to bedtime.
It is also thought that changes in melatonin levels can provide information about changes
in night length, which may provide the brain with information about seasons.
In some animals, this information may help to regulate seasonal changes in things like
reproduction, hibernation, and migration.
Melatonin is believed to have a number of other functions as well.
It is thought, for example, to modulate immune system activity and to act as a free radical
scavenger and antioxidant.
Two G-protein coupled melatonin receptors, MT1 and MT2, have been identified and found
throughout the body.
Melatonin also seems to bind to other sites, although the details of these mechanisms are
still being investigated.
Additionally, some of melatonin’s effects (like its antioxidant effects) are thought
to be exerted independently of receptors.