Melatonin and human sleep
What is melatonin?
The diurnal rhythm of melatonin production is almost entirely controlled by the body clock. Your body's natural melatonin levels don't make your baby sleep, and don't control your baby's body clock! The melatonin produced naturally by the human body is a follower, not an instigator, when it comes to sleep: your melatonin levels track your body clock settings. The body clock itself is predominantly controlled by the sun and our environment's cycles of light and dark.
Melatonin is a neurohormone which is secreted into the blood stream by the cells of the pineal gland. Many other parts of the body also secrete melatonin. Melatonin helps synchronise the suprachiasmatic nucleus (SCN) or master pacemaker with the myriad cellular body clocks in tissues throughout the human body.
Why does daylight put the brakes on melatonin secretion?
Light sensed by the inner retinal ganglion cells in the human eye causes nerve signals which travel into the brain, ending up in the
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Visual areas, giving us the capacity to see
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Suprachiasmatic nucleus (SCN), also known as the master pacemaker or body clock
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Pineal gland. (Only a very small number of retinal cells send signals directly to the pineal gland).
When light signals reach it, the SCN or body clock secretes a neurohormone which puts a brake on certain nerve cells in the hypothalamus, which in turn inhibit the pineal gland so that melatonin isn’t synthesised. This is why melatonin levels are low or undetectable during the day.
When there are no light signals, the body clock secretes another neurohormone which stimulates certain nerve cells in the hypothalamus. These nerve cells in turn stimulate the pineal gland, so that melatonin synthesis continues. This is why melatonin levels peak during the night, between two and four o'clock in the morning.
Light exposure (or lack of it!) is the most important driver of pineal gland function and melatonin secretion. This is why melatonin levels peak between two and four o'clock in the morning. A molecule of melatonin only lasts in the blood stream for about 40 minutes, targeting cells inside the brain and throughout the body. It's then metabolised in the liver. So when daylight comes, and synthesis ceases, melatonin quickly disappears from the blood stream.
What does the latest research tell us about melatonin and sleep?
There are five main things we can say with confidence from the research about melatonin and and how it interacts with the circadian system and sleep patterns.
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Melatonin levels can be measured in the blood plasma to work out whether the internal body clock is in a night or daytime phase.
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Giving someone a dose of melatonin (noting that effective doses have not been found or agreed upon) elevates melatonin plasma levels, both during the night, and also during the day when normal melatonin levels are usually undetectable.
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Giving someone a dose of melatonin also alters other factors influenced by body clock settings. For example, changes in plasma thyroid stimulating hormone, cortisol, and core body temperature are brought forward in time.
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Melatonin plays only one small part of the body’s complex system of sleep regulation. Despite what you might hear, the research shows that even a supplemental dose of melatonin which dramatically increases plasma levels may not make much difference, actually, to an adult's sleep.
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The safety profile and efficacy of longer-term use of melatonin supplements requires further study.
You can find a table which details the science as it stands right now, showing why melatonin follows the body clock, not the other way around, here.
You can see that the melatonin science doesn't support the idea that you can bring on your own or your child’s sleep by dimming the lights in the evening! Advice to dim the lights in the evenings to increase your baby's melatonin and make him sleep is an example of what we might call the One Factor Fallacy.
Recommended resources
Will dim lights in the evening increase melatonin and make sleep easier for your baby?
Will dim lights in the evening increase melatonin and make sleep easier for your toddler?
Can giving baby your expressed breast milk be 'mistimed' because of melatonin levels?
Melatonin: production in babies (term + preterm) and supplementation
Melatonin research doesn't support the belief that dimming the lights helps with infant sleep
Selected references
Givler D, Givler A, Luther PM. Chronic administration of melatonin: physiological and clinical considerations. Neurology International. 2023;15:518-533.
Tordjman S, Chokron S, Delorme R. Malatonin: pharmacology, functions and therapeutic benefits. Current Neuropharmacology. 2017;15(434-443).
