The importance of sunlight
In a nutshell
Sunlight has benefitted microbes, plants, and animals since life began
Sunlight is good for our skin, our muscles, and the bones and organs inside us
Isolating ourselves from sunlight is detrimental to our health…maybe fatally
It’s August 2024 as I write this and my time in the sun gets less each day now. Conversely, I know that my southern hemisphere readers (I’m certain there’s one) are experiencing an increase in their daily exposure. This year I’ve made a conscious effort to get as much sun as possible now that my no seed oil lifestyle means I don’t burn in the sun the way I used to.
“…when we can harvest the light that reaches the surface of the earth, we are functioning as we are adapted to as human beings”
I’ve been reading extensively about sunlight and its health benefits and the things I’ve learned are so different from what I believed when I started. I last felt my worldview change so dramatically when I learned in 2017 that a recommended western diet and lifestyle caused my heart disease.
I’m still coming to terms with what I’m reading. However, it does appear to be the case that when we can harvest the light that reaches the surface of the earth, we are functioning as we are adapted to as human beings [1].
I’ll get the ball rolling with this as a scene-setting introduction and provide updates as my thinking develops.
In this article I’ll cover the following:
The nature of sunlight
Sunlight’s evolutionary importance
The beneficial effect of sunlight for human health
The Nature of sunlight
Let’s get one fancy term out of the way, namely, electromagnetic radiation (EMR). Looking at Figure 1, it can be seen that the sun emits radiation with a wide range of different wavelengths and energy. When EMR has short wavelengths, it has high energy. For example, cosmic and gamma radiation are the shortest wavelength EMR and the highest energy. Conversely, radiowaves have much longer wavelengths and much less energy [3].
I have often associated radiation with biological damage. For example, gamma radiation which is emitted by nuclear bombs as well as the sun, is damaging if it reaches the human body. However, most of the sun’s radiation that reaches us is beneficial.
Figure 1: The spectrum of EMR emitted by the sun highlighting the proportion reaching the human body and the small range we perceive as visible light. Adapted from reference 1
Sunlight making it through the earth’s atmosphere
Figure 1 shows that most of the sun’s EMR reaching earth lies in the range from ultraviolet (UV), through visible to infrared (IR). We tend to refer to those as forms of light, so from here on I’ll simply refer to sunlight or light when describing the effects of UV, visible or IR.
Tissue penetration
Sunlight penetrates our skin according to its wavelength [1]. Shorter wavelengths such as UV penetrate just the upper surface of skin, whereas longer wavelength IR gets deeper into our bodies. Visible light penetration lies in between those two extremes (Figure 2).
Figure 2: Sunlight enters the human body to varying degrees. UV is very shallow and penetrates only the surface of skin. IR gets deep into human tissues and their mitochondria
Human Evolution and Sunlight
For millions of years sunlight has been the single greatest environmental influence on the human body [2]. Unsurprisingly, the human body has evolved to put sunlight to good use.
Humans are adapted to use the spectrum, and not simply the individual elements within the spectrum, as biological signals within our bodies [1]. Because sunlight is always changing in time (daily and seasonally) and location (e.g., the equator versus Scotland), changes in the nature of the spectrum help our body to function efficiently in any given environment.
I was convinced that the human species needs sunlight when I considered what happened when our ancestors migrated from their sunny tropical origins to places like Scotland where it is not always (ahem) so sunny. As they migrated northward, they experienced lower and lower sun exposure. Those who retained their originally protective dark skin at some point suffered from a lack of sunlight. Conversely, those of our ancestors whose skin lightened and allowed more sunlight to contact and penetrate our skin were better able to survive and reproduce in the lower sun environment. They then passed on their lighter skinned characteristics down the generations. I can think of no stronger evidence for the notion that human health depends on adequate exposure to sunlight.
The beneficial effect of sunlight for human health
Importance of the spectrum
The key to understanding how sunlight helps us begins with recognizing that the various components of light work together beneficially in ways that cannot be achieved if we experienced the same components of sunlight individually [1-6]. In other words, individual components of sunlight work together synergistically to the benefit of our health.
This synergistic quality is very similar to the way that we benefit from real food that contains many different micronutrients and from a microbiome containing diverse microbiota (fungi, archea, and bacteria) acting synergistically to our benefit.
Lets consider what is known about the spectrum of light that reaches us.
Ultraviolet
Ultraviolet light is the highest energy wavelength of the spectrum that routinely reaches the earth’s surface [1]. Safe exposure to UV provides a range of benefits to our metabolism, immunity, and microbiome [1].
I have discussed before how UV reacts with cholesterol in the human skin to produce the important hormone Vitamin D. In addition to the better-known benefits to our metabolism and immune system, Vitamin D helps protect skin cells from potential UV damage and melanoma.
UV exposure also causes better blood flow, lowers blood pressure, and improves circulation. These heart health benefits are thought to be caused by the release within our body of a gas called nitric oxide.
There appear also to be indirect effects of UV on human health through its interaction with the human microbiome. Firstly, UV exposure causes us to produce compounds (aromatic amino acids) that trap even more UV. Secondly, UV exposure results in the production of things that make us feel good (endorphins) and improve brain function. The extent to which these things happen to the human body directly or result from interaction with our gut microbiome, are still unclear to me. Irrespective, UV exposure results in improved cognition, mood, relaxation, pain relief, and immune function.
It appears that UV also directly improves the human skin microbiome.
Interestingly, UV-induced release of endorphins may represent an in-built addiction to sun exposure. As sun exposure increases, we have more vitamin D and the need for more sun decreases.
Visible
Visible light is something that our eyes have evolved to detect [1] (Figure 1). It can be divided into the seven colours of the rainbow we are taught about from an early age, ranging from violet through indigo, blue, green, yellow, and orange to red.
Most of the information I’ve found about the visible spectrum of sunlight relates to blue and red light. The information on violet to orange is patchy and inconclusive.
Blue light is the highest energy form in the visible spectrum and it is present for most of the day in which we see the sun. Exposure to blue light starts as soon as the sun appears in the morning, increases until it peaks in the middle of the day and then decreases towards nighttime. We sense blue light through our eyes and skin, and it has two main impacts:
It provides the most powerful signal to set our circadian rhythm each day and stimulates hormones (e.g., cortisol) which provide full wakefulness in the morning
During the day it stops the production of melatonin by our pineal gland. As blue light decreases at night, our pineal gland releases melatonin into our blood and we start to feel sleepy
Red light reaches earth throughout the day and is absorbed by the human body through the skin (Figure 2). Red light can penetrate clothing. The cells in and below our skin benefit from red light because it helps our mitochondria to produce energy more efficiently.
“IR is the most abundant form of light reaching earth constituting around 50% of the total, and provides “the primary stimulus during the day to the majority of cells in the human body””
Infrared
IR is the most abundant form of light reaching earth constituting around 50% of the total, and provides “the primary stimulus during the day to the majority of cells in the human body”. We typically perceive IR light as heat, as for example, when we feel full sunshine or when we sit around a campfire.
There are three important things to understand about IR:
• It penetrates deeply into the human body
• It is reflected the by plants around us
• It is the most important source of melatonin
Just as with red light, IR can penetrate clothing and bodily tissues. However, IR gets much deeper into the human body (Figure 2) and can go through soft tissue and bone to reach important organs and our central nervous system including brain and the fluid that envelops the brain and spine (cerebro-spinal fluid).
“…IR impacts 60% of the cells in an adult human and 100% of the cells in the developing fetus and young children.”
IR also helps our mitochondria to produce energy more efficiently. However, because it reaches more of our body, IR is even more beneficial to human biology than red light. To put this in perspective, IR impacts 60% of the cells in an adult human and 100% of the cells in the developing fetus and young children [2]. Moreover, the human body has evolved ways to gather IR in some of its most sensitive areas, including blood vessels, eyes, skin, brain, and the developing fetus.
“…when in nature, we are bathed in IR that comes directly from the sun, from atmospheric scatter, and from the plants around us.”
The plants around us, whether in a forest, a field, or a back garden strongly absorb visible light for photosynthesis but reflect IR [5]. It’s difficult to find out how much is reflected but at a minimum appears to be 50% [6]. This means that when in nature, we are bathed in IR that comes directly from the sun, from atmospheric scatter, and from the plants around us [5].
“Melatonin is the most important antioxidant in the human body”
As if finding out that IR reaches us from the sun and surrounding plant life and that it penetrates quite deeply into the human body wasn’t enough, I was blown away by reading more about the hormone melatonin [2]. The human pineal gland releases melatonin at the end of the day at which point our blood circulates the hormone around the body to signal that it’s time to go to sleep. However, it turns out that melatonin is more than a sleep signal, much more.
Melatonin is the most important antioxidant in the human body [2]. Let’s remind ourselves what is meant by antioxidant. We need antioxidants to neutralize the waste products we produce every moment of every day through normal human metabolism. I think of antioxidants as like car exhaust catalytic converters getting rid of the nasty stuff produced by an engine as it moves a car along. Vitamin C is promoted as an important antioxidant, and we’re encouraged to take a Vitamin C supplement daily.
Melatonin is now thought to be produced in two ways. The first, as discussed above, is by the pineal gland as the sun disappears and before it re-appears in the morning. It is thought that this circulating source of antioxidant has evolved as a supplement to the main supply during sleep, when there is no sunlight, and our metabolic rate is lowered.
The main source of melatonin by far is produced during the day as IR penetrates the human body and stimulates production in individual cells. Melatonin is thus produced in each cell according to its individual metabolic need and does not enter the circulatory system. This IR-induced melatonin production has the advantages of rapidly responding to local cellular conditions as our metabolism fluctuates throughout the day [2,4].
Another benefit of IR-induced melatonin production occurs in skin where it is thought to guard against UV-induced damage and sunburn.
“Our world is dominated by sunlight...we just happen to live in it. Isolating ourselves from that world is detrimental to our health…maybe fatally so..”
How I get enough sunlight…safely…and avoid disruption
I’ll cover more of this in my next article on sunlight. Here’s a summary:
Maximum exposure to the full spectrum to get the synergistic benefits
Short sessions in the sun, just as with exercise, to gauge my body’s reaction
Exposure to sunlight throughout the day – especially early morning
Protection against the POTENTIAL for UV harm
No dietary seed oils – I avoid always for any reason
No 100% use of high protection factor sunscreen
Low protection factor sunscreen on my face
One-hour sessions in full mid-day sunlight
Use full-length clothing and a hat if necessary
Sit in shade – under a tree is best for reflected IR
Protection against potential harm of too little sunlight in Scotland
Ongoing vitamin D supplement
Maximum, comfortable bare skin exposure – bike, run, garden, etc.
IR lamp sauna use
Protection against exposure to unnatural EMR
Limit smart-phone and computer use before early-morning sun exposure
No smart-phone and computer use in bed
Use fuller-spectrum light bulbs and fewer LEDs in the house
Use red-light spectrum light bulbs in bedrooms, patios, etc.
Summary
It has been suggested that “sunlight is the most important nutrient for our body” [1]. I’m not on board with the notion that sunlight is a nutrient. However, describing it as such is appropriately attention grabbing, under the circumstances.
I’m reminded of the article I wrote about food not being medicine, it is much more important than that. That was the first time I realized that my framework for understanding human nutrition was not up to the task. The nutrition we derive from our food is a bedrock evolutionary need whereas medicine is something we’ve recently invented to fix the malnutrition caused by not eating proper food.
Learning about the role of sunlight highlighted for the second time the inadequacy of my human health framework. Sunlight has benefited microbes, plants, and animals since the beginning of life on earth. Our world is dominated by sunlight…we just happen to live in it. Isolating ourselves from that world is detrimental to our health…maybe fatally so.
Before I leave you alone, the most important thing I did to ensure a healthy relationship with sunlight was to minimise my exposure to industrially processed seed oils.
References
Why You Can Never Replace Full Spectrum Sunlight https://tristanhealth.substack.com/p/why-you-can-never-replace-full-spectrum
Zimmerman, S. and Reiter, R. 2019. Melatonin and the Optics of the Human Body. Melatonin Research. 2, 1 (Feb. 2019), 138-160. DOI:https://doi.org/https://doi.org/10.32794/mr11250016
Light is an Essential Nutrient https://www.midwesterndoctor.com/p/why-light-is-an-essential-nutrient
Tan DX, Reiter RJ, Zimmerman S, Hardeland R. Melatonin: Both a Messenger of Darkness and a Participant in the Cellular Actions of Non-Visible Solar Radiation of Near Infrared Light. Biology (Basel). 2023 Jan 6;12(1):89. doi: 10.3390/biology12010089. PMID: 36671781; PMCID: PMC9855654.
Photobiology, Sunlight, Firelight, Incandescent Bulbs vs. LEDs, Mitochondria, Melatonin, Sunscreen & the Optics of the Body https://mindandmatter.substack.com/p/photobiology-sunlight-firelight-incandescent
More than meets the eye https://medium.com/untill-ag/more-than-meets-the-eye-using-light-reflectance-to-monitor-crops-indoors-446c5a550c4d