What is Melatonin?
What is melatonin?
The production and release of melatonin from the pineal gland occurs with a clear daily (circadian) rhythm, with peak levels occurring at night. Once produced, it is secreted into the blood stream and cerebrospinal fluid (the fluid around the brain & spinal cord) and conveys signals to distant organs. 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. Night-time levels of melatonin are at least 10-fold higher than daytime concentrations.
In addition to its circadian rhythm, melatonin levels also have a seasonal (or circannual) rhythm, with higher levels in the autumn and winter, when nights are longer, and lower levels in the spring and summer.
Most of the research into the function of the pineal gland involves the human brain's responses to mevlatonin rhythms. The evidence supports two roles for melatonin in humans: the involvement of nocturnal melatonin secretion in initiating and maintaining sleep, and control by the day/night melatonin rhythm of the timing of other 24-hour rhythms. Melatonin has, therefore, often been referred to as a ‘sleep hormone’; although it is not essential for human sleep, we sleep better during the time that melatonin is secreted.
Night-time melatonin secretion is suppressed by a relatively dim light when pupils are dilated. This has been suggested as the main way through which prolonged use of devices such as laptops and smartphones before bedtime can have a negative impact on melatonin secretion, circadian rhythms and sleep.
How is melatonin controlled?
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.