Melatonin is produced by various tissues in the body, although the major source is the pineal gland in the brain. The production and release of melatonin from the pineal gland occurs with a clear daily (circadian) rhythm, with peak levels occurring at night. Melatonin is carried by the circulation from the brain to all areas of the body. Tissues expressing proteins called receptors specific for melatonin are able to detect the peak in circulating melatonin at night and this signals to the body that it is night-time. The level of circulating melatonin can be detected in samples of blood and saliva, and this is used in clinical research to identify internal circadian rhythms.
In many animals (including a wide range of mammals and birds), melatonin from the pineal gland is essential for the regulation of the body’s seasonal biology (e.g. reproduction, behaviour and coat growth) in response to changing day length. The importance of pineal melatonin in human biology is not clear, although it may help to synchronise circadian rhythms in different parts of the body. Melatonin has often been called a ‘sleep hormone’ - although it is not essential for human sleep, we sleep better during the time that melatonin is secreted.
Association between tumours of the pineal gland and the timing of puberty suggests that melatonin may have a minor role in reproductive development, although the mechanism of this action is uncertain.
In addition to its production in the body, melatonin can also be taken in capsule form. When administered at an appropriate time of day, it can reset the body’s circadian rhythms (see the articles on jet lag and circadian rhythm sleep disorders). This resetting effect of melatonin has been reported for many dose strengths, including those that are equivalent to the concentration of melatonin naturally produced by the pineal gland. Higher doses of melatonin can reset circadian rhythms, bring on sleepiness and lower core body temperature.
In humans and other mammals, the daily rhythm of pineal melatonin production is driven by the 'master' circadian clock. This 'clock' is in a region of the brain called the suprachiasmatic nuclei, which expresses a series of genes termed clock genes that continuously oscillate throughout the day. This is synchronised to the solar day via light input from the eyes. The suprachiasmatic nuclei link to the pineal gland through a complex pathway in the nervous system, passing through different brain areas, into the spinal cord and then finally reaching the pineal gland. During the day, the suprachiasmatic nuclei stops melatonin production by sending inhibitory messages to the pineal gland. At night however, the suprachiasmatic nuclei are less active, and the inhibition exerted during the day is reduced resulting in melatonin production by the pineal gland.
Light is an important regulator of melatonin production from the pineal gland. Firstly, it can reset a specific area of the brain (the suprachiasmatic nuclei clock) and, as a result, the timing of the melatonin production. Secondly, exposure to light during the body's biological night reduces melatonin production and release.
There are large variations in the amount of melatonin produced by individuals and these are not associated with any health problems. The main consequences of swallowing large amounts of melatonin are drowsiness and reduced core body temperature. Very large doses have effects on the performance of the human reproductive system. There is also evidence that very high concentrations of melatonin have an antioxidant effect, although the purpose of this has not yet been established.
Reduced melatonin production is not known to have any effect on health.
Last reviewed: Jan 2015