The hormone melatonin has traditionally been regarded as the hormone of darkness, because of the nightly surge in production that is associated with the sleep cycle. It is also associated with seasonal cycles.
Research over the past two decades found high levels of melatonin in the body during daytime, far too high levels to have come from the nightly production of melatonin by the tiny pineal gland in the brain. Scientists also found more sources of melatonin in the body, with one of them linked to sunlight.
What is melatonin?
Melatonin is a neurohormone that is commonly known to prepare the body for sleep. It plays a crucial role in regulating the body’s circadian rhythm, also known as the biological clock, in the 24-hour sleep-wake cycle. Melatonin also regulates a seasonal rhythm, with higher levels of melatonin in the autumn and winter when nights are longer, and lower levels in spring and autumn when nights are shorter.
Circulatory melatonin, the hormone of Darkness:
Nighttime production of melatonin results from the detection of light and dark by the retina of the eye, with the production of melatonin inhibited during the light of day and stimulated in the dark of night. Photoreceptor cells in the retina send light or dark signals along the optic nerve to the suprachiasmatic nucleus (SCN) in the hypothalamus of the brain, from where these signals are transmitted to the pineal gland. The pineal gland is a tiny endocrine gland that is situated near the center of the brain. The suprachiasmatic nucleus is the brain’s master clock, which controls the production of melatonin and other hormones in the 24-hour cycle of day and night.
When activated at night, the pineal gland secretes melatonin into the bloodstream as well as into the cerebrospinal fluid around the brain and spinal cord, from where it is carried to all areas of the body. Receptors in the cells and tissue of the body detect the peak in melatonin being circulated at night and signals to the body that it is time to sleep. Melatonin levels from the pineal gland typically begins to rise around dusk and peak during the night at around 02:00, before falling to very low levels shortly before dawn. The pineal gland secretes melatonin between 7,5 and 8 hours at night and helps to promote sleep by assisting the body’s master clock in regulating sleep-wake cycles, resulting in healthy sleeping patterns.
Around sunrise the retina starts to detect large amounts of light and the production of melatonin in the pineal gland is suppressed. Although melatonin is not essential for sleep, we sleep better during the time that circulatory melatonin is secreted by the pineal gland.
Subcellular melatonin, the hormone of Sunshine:
The regulatory mechanisms of melatonin in the pineal gland apply to visible light in the range of 400 nm to 700 nm (nanometres), particularly the blue wavelengths. However, we are exposed to solar spectral wavelengths from 250 nm to over 4000nm, with blue light representing only about 2% of the spectrum emitted by the sun and it only penetrates the surface of the body.
The non-visible near infrared radiation (NIR) portion of sunlight, between 650 nm and 1200 nm, interacts with a high percentage of the body’s cells, as NIR light penetrates the skin, underlying tissues, and organs to a depth of several centimeters, for example muscles, blood, and even the brain.
The actions of melatonin at mitochondrial level have primarily been identified within the past two decades, with many of the research findings within the last several years. Findings indicate that melatonin is produced and has been detected in the mitochondria of many cells, some researchers reckon in all cells in the human body.
NIR photons penetrating the mitochondria in cells result in improved mitochondrial energy metabolism and have been linked to increased melatonin production in mitochondria. The subcellular generated melatonin has limited access to the bloodstream and is deemed to be used in the cell of origin to protect the cell against oxidative stress.
Melatonin is known as a major antioxidant in the body’s cells, including brain cells (neurons). Daily exposure to sunlight is deemed to boost the brain’s antioxidant capacity by increasing melatonin production in brain cells due to the deep penetration of NIR in the cerebrospinal fluid in the brain during the day, while relying on pineal produced melatonin as antioxidant at nighttime.
Subcellular produced melatonin is deemed to be of a magnitude much higher than the amount of pineal-generated melatonin, as the rather small quantities of melatonin secreted by the pineal gland would be inadequate to cater for the trillions of mitochondria.
The latest research suggests that melatonin is at the core of human health and the Sun has been exposing our bodies to NIR for billions of years. For hundreds of years groups of people were exposed to the sun during the day and at night again exposed to NIR when gathered around the campfire, with NIR emitted by the flames. Green spaces like when out in nature, also scatters NIR light, as well as in early mornings and early evenings. NIR exposure also includes light from candles and incandescent bulbs, which have nowadays been largely replaced by fluorescent and LED bulbs.
Interestingly, the scalp and skull block harmful UV photons from the sun, but transmits NIR photons into the cerebrospinal fluid, which acts as a light guide to distribute the NIR photons even deep into the folds of the brain. Although more research in this area is needed, daylighting studies have shown that NIR-rich natural sunlight improves the learning rates of children compared to NIR-deficient artificial lighting, with melatonin playing an important role in this regard.
Other sources of melatonin:
At least four different sources of melatonin contribute to the melatonin pool in humans. These are:
- Visible-light regulated melatonin originating from the pineal gland, circulating in the cerebrospinal fluid and the bloodstream at night..
- NIR-regulated melatonin, the largest stimulus in the production of melatonin (95%).
- The microbiome, consisting of, for example, the microbiota of microorganisms in the gut, the skin, the respiratory tract, and even the vagina, is deemed to also make melatonin.
- As melatonin is present in plants and animals, the body also obtains melatonin from dietary sources. (tart cherries, goji berries, eggs, milk, fish, nuts)
Free radicals and the role of antioxidants in the body’s cells:
NIR-made melatonin acts as a major antioxidant and helps to protect cells in the body from damage from free radicals. What does this mean?
- Free radical: A type of unstable molecule made during normal cell metabolism. They can build up in cells and cause damage to other molecules, such as DNA, lipids, and proteins. Damage from free radicals may increase the risk of cancer and other diseases.
- Antioxidant: This is a substance that protects cells from the damage caused by free radicals, by scavenging free radicals.
The subcellular distribution of melatonin and its high concentrations in mitochondria suggest that melatonin could be classified as a mitochondria-targeted antioxidant, with free radical scavenging and anti-inflammatory effects.
Conclusions:
Modern technology in lighting, electronic devices, and architecture has resulted in us spending the largest amount of our time under artificial lighting and in front of displays that emit zero NIR. In effect, modern society has created NIR-lacking indoor caves and about 70% of the spectrum emitted by the Sun is being eliminated from our lives during the day.
The light from our electronic devices has a higher concentration of blue light than natural light and affects the levels of melatonin (inhibits) more than any other wavelength. Studies have shown that exposure to blue light a couple of hours before bedtime suppresses the production of melatonin and delays deep sleep in the REM phase significantly.
The modern environment and lifestyle may well have an adverse effect on the production of melatonin, affecting biochemical processes in our bodies and our health.
References:
Melatonin and the optics of the human body. Published 21 February 2019. Melatonin Research. (www.melatonin-research.net)
Melatonin and pathological cell interactions: Mitochondrial glucose processing in cancer cells. Published online 19 November 2021 and in International Journal of Molecular Sciences. 2021 Nov, 22(22); 12494. (www.ncbi.nlm.nih.gov)
Melatonin. Published online and reviewed November 2021. You and your Hormones. An education resource from the Society for Endocrinology. Bristol. United Kingdom. (www.yourhormones.info)
Melatonin: Both a messenger of darkness and a participant in the cellular actions of non-visible solar radiation of near infrared light. Published 6 January 2023 in the journal Biology, published by the Multidisciplinary Digital Publishing Company (MDPI). Based in Basel, Switzerland. (www.mdpi.com)
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