Melatonin is mainly secreted from the pineal gland at night. It is also secreted in small amounts from the retina. We know that the level of melatonin secreted varies depending on the time of day or night. In this post we’ll explore the pattern of melatonin secretion in more detail.
The release of melatonin from the pineal gland is stimulated by the onset of darkness, and it is inhibited by exposure to light. The secretion of melatonin peaks in the middle of the night, usually between 2 AM and 4 AM, and it falls during the second half of the night. Here is a graph illustrating this1:
This graph1 shows that in humans, melatonin has diurnal variation. Diurnal is a term used in the field of biology to describe something that has a daily cycle, and is happening at recurring or regular intervals.
Given melatonin’s sensitivity to light, its pattern of secretion is affected by seasonal changes. There are increased levels and duration of secretion of melatonin in the winter months. In the summer months the opposite happens, lower levels of melatonin are secreted, and the secretion of melatonin occurs for a shorter period of time2. This can help explain why many of us sleep for longer during the dark winter months compared to the summer.
There is a pathway called the retino-hypothalamic tract (RHP). This pathway includes the pineal gland, photoreceptor cells in the retina (called melanopsin retinal ganglion cells), and circadian pacemaker cells in the suprachiasmatic nucleus (SCN) of the hypothalamus3.
In many individuals, the retino-hypothalamic axis is capable of detecting changes in the length of the night and making proportional adjustments in the duration of nocturnal melatonin secretion2.
The level of melatonin varies depending on age. Babies below the age of 3 months secrete very little melatonin. The secretion of melatonin gradually increases in older infants, with peak nocturnal concentrations between the age of 1-3 years. After this, peak nocturnal concentrations of melatonin gradually decrease by roughly 10-15% per decade4.
Here is a graph showing the changes in the level of melatonin secretion across different age groups1:
Newborn babies produce very small amounts of melatonin. The secretion of melatonin increases and peaks in early childhood. As puberty occurs, melatonin secretion decreases. Melatonin may play a role in the timing of puberty and the onset of puberty in humans may be related to the decline in melatonin secretion that occurs as children grow5. Melatonin secretion continues to gradually decline in middle-age. In old age, melatonin levels are negligible. Concentrations of melatonin in elderly people with insomnia are significantly lower compared to people of the same age without insomnia6.
References:
1. Grivas TB, Savvidou OD. Melatonin the “light of night” in human biology and adolescent idiopathic scoliosis. Scoliosis. 2007 Apr 4;2:6. doi: 10.1186/1748-7161-2-6. PMID: 17408483; PMCID: PMC1855314.
2. Wehr TA. Melatonin and seasonal rhythms. J Biol Rhythms. 1997 Dec;12(6):518-27. doi: 10.1177/074873049701200605. PMID: 9406025.
3. Vosko, Andrew & Colwell, Christopher & Avidan, Alon & Alon, Correspondence & Avidan, Alon. (2010). Jet lag syndrome: Circadian organization, pathophysiology, and management strategies. Nature and Science of Sleep. 2. 2-187. 10.2147/NSS.S6683.
4. Waldhauser F, Weiszenbacher G, Frisch H, Zeitlhuber U, Waldhauser M, Wurtman RJ. Fall in nocturnal serum melatonin during prepuberty and pubescence. Lancet. 1984 Feb 18;1(8373):362-5. doi: 10.1016/s0140-6736(84)90412-4. PMID: 6141425.
5. Haimov I, Laudon M, Zisapel N, Souroujon M, Nof D, Shlitner A, Herer P, Tzischinsky O, Lavie P. Sleep disorders and melatonin rhythms in elderly people. BMJ. 1994 Jul 16;309(6948):167. doi: 10.1136/bmj.309.6948.167. PMID: 8044096; PMCID: PMC2540689.
6. Haimov I, Laudon M, Zisapel N, Souroujon M, Nof D, Shlitner A, Herer P, Tzischinsky O, Lavie P. Sleep disorders and melatonin rhythms in elderly people. BMJ. 1994 Jul 16;309(6948):167. doi: 10.1136/bmj.309.6948.167. PMID: 8044096; PMCID: PMC2540689.
