It is dark inside your head

But even there, the sun does shine

Your thoughts are born in darkness. All the biology and chemistry and electricity that comprise your brain, exist in darkness.

Visible light does not pass through your skull or into your brain. The motor messages going out—move that, go there—and the sensory messages coming in—smell this, feel that—occur in darkness. So too, do reflections on the past, predictions of the future, plotting and planning, analysis and logic, storytelling and imagining. All happen where the sun doesn’t shine.

Except that that last part is not actually true.

It is dark inside your head, but even there, the sun does shine.

Visible light and sunlight are not synonyms. Visible light, as anyone who has looked at a graphic of the electromagnetic spectrum will remember, is but a sliver of all the light available, a small fraction of all the light pouring forth from the sun.

Off the long wavelength end of the visible spectrum, past red, there is infrared. On the opposite side of the visible spectrum, beyond the edge of violet into that which we cannot see, we find ultraviolet (UV).

We do not see either infrared or ultraviolet. Some other animals on this beautiful planet of ours can, however. Pit vipers, for instance—which includes rattlesnakes, copperheads, and bushmasters—have infrared sensing organs (“pits”) on the sides of their heads, which enable them to locate warm-blooded prey in the dark of night. And mammals, of which we are but one of thousands of species, are unusual among animals in not being able to see ultraviolet.¹ Birds, for instance, have four kinds of cones in their eyes to our three, the fourth type allowing them to see into the near ultraviolet. Bird plumage, often extravagant even to our eyes, looks even more impressive to other birds.

Many other animals see into ranges beyond what we can see, but we are the ones with the naming rights. Because we see what we see, and we are the ones with language, we call our range the range of visible light.

And even if you’re a copperhead or a cardinal, and you can see things that we can’t, the inside of your head is still dark. Visible light does not penetrate the brain. Same with ultraviolet. Those rays are nearly fully blocked by the barriers of our bodies—by our skin, and even more so by our skulls. Ultraviolet and visible photons hit our surfaces and allow us to make vitamin D and to see our world. But they don’t go farther than that. Our kidneys do not receive visible or ultraviolet light, nor our brains. Our insides are dark.

But the majority of photons that reach us on Earth are not in the visible spectrum, nor in the ultraviolet, but in the Near Infrared (NIR).² NIR is light that we cannot see, so even in its presence, if there is no visible light, we say that it is dark out. This is a confusing linguistic convention, but it is what it is.

Near Infrared (NIR) photons, unlike those of visible light or UV, penetrate deep inside our bodies, even into our internal organs. Babies and young children are small enough that their entire bodies are perfused with NIR. Adults are bigger, thicker, so NIR doesn’t penetrate all of our parts. Only about 60% of the cells of an average adult receive NIR from the sun.³

Not just our bodies, but our brains, are bathed in NIR photons—again, in photons from the sun that we cannot see. In fact, the liquid that runs through our Central Nervous System, the cerebrospinal fluid, seems optimized to efficiently distribute NIR photons deep into the brain. Indeed, the automotive and avionics industries use bezels and faceplates to direct and distribute photons in much the same way that cerebrospinal fluid does in vertebrate brains.

“As such the brain appears to be optically designed to distribute NIR photons to the grey matter even down into the folds of the brain.”⁴

The inside of your head is dark, but the sun’s rays do get in.

Why, though? Why does the cerebrospinal fluid seem to be adapted to send NIR deep into our brains? What is NIR doing for us?

Near Infrared photons have been discovered to be useful in treating a range of ailments, from soft tissue injuries and wound healing, to macular degeneration and, most recently, Alzheimer’s disease.⁵ Research in this area is expanding rapidly, and much is yet unknown about the mechanisms of action of NIR. One powerful and well-supported hypothesis is that NIR promotes the synthesis of subcellular melatonin.⁶

What then, in turn, is important about subcellular melatonin?

For decades, scientists and health advisors have been focused on circulating melatonin, which is produced by the pineal gland. The public has been told that (circulating) melatonin is the hormone of sleep, and that having it in sufficient amounts helps entrain our sleep/wake cycles, keeping our circadian rhythms humming along. But the subcellular melatonin that seems to be produced in the mitochondria of our cells by exposure to NIR is far more abundant in our bodies than is circulating melatonin, and has functions far beyond the entraining of circadian rhythms.

Subcellular melatonin is a powerful antioxidant. In fact, melatonin’s original evolutionary function was as an antioxidant; its ability to entrain circadian rhythms came later.⁷

Oxidation is a ubiquitous and destructive process in organisms, a natural side-effect of metabolism. Antioxidants scavenge the free radicals formed by oxidation, thus mitigating its deleterious effects. Melatonin is a particularly strong and multi-functional antioxidant, and is even known to control other antioxidant systems in the body.⁸

In the brain, melatonin’s antioxidant capacity is likely to be especially important, both because of the disproportionately high use of oxygen by the brain, and because some other antioxidant systems are not present there.⁹ Absent near infrared photons being carried deep into the brain, which then promote the synthesis of subcellular melatonin, brain function might go more than a bit haywire.

Subcellular melatonin also reduces inflammation, increases the rates of waste removal within cells, and has immunoregulatory effects. Furthermore, it improves mitochondrial function via several known mechanisms,¹⁰ and generally promotes health and vitality. And again, there is substantial evidence that NIR prompts its formation.

The sun emits NIR photons at a staggering rate. Moonlight does as well, at a rather less substantial rate. So does firelight. Even incandescent bulbs do. But compact fluorescent bulbs, and the LEDs that we currently have access to, do not.¹¹ So those of us who spend most of our time inside, with little exposure to sunlight, working in “modern” spaces without candles or incandescent bulbs, are getting very little NIR. Our cerebrospinal fluid can’t direct NIR deep into our brains if we aren’t receiving any in the first place. The brains of those who spend all their time inside are both dark, and without any sunshine at all. And that lack of sunshine in the brain may be making us very sick indeed.

It was an evolutionary given for billions of years that we would be bathed in the sun’s rays throughout much of every day.

The sun came before everything you know. It came before you, and before everyone in your family. It came before all life on Earth, and before the Earth itself. When life began, the sun was already a constant. Everything we are, all of our systems, assume that sunlight is a large part of our lives.

Now we spend most of our time inside, lit by lights that cast photons only in the visible spectrum, a narrow band of what the sun produces. We are coming to know some of what that is costing us, health wise. Presumably, there is much more that we do not yet know.

In 1914, shortly before becoming a Supreme Court Justice, Louis Brandeis wrote that sunlight is the best disinfectant.¹² He was speaking metaphorically, but it is literally true. Sunlight does disinfect.

But sunlight is not merely disinfectant, it is an active healer. And some parts of sunlight—the near infrared photons on which we have been focused here today—penetrate clothing, and reflect off leaves. Shortly after sunrise, and before sundown, the availability of near infrared photons is particularly high. Just being outside in the world, even under light cover, earns you many of the health benefits of the sun.

So every day, early and often, get outside.

Related content:

• Hospitals Should Let the Outside In: It’s remarkable what fresh air and sunlight can do for your health. (Natural Selections essay #3, on August 3, 2021)

• Vitamin D Deficiency and Covid-19: Make D while the sun shines (And when the sun doesn’t shine, supplement) (Natural Selections essay #15, on October 26, 2021)

• Compelled Speech Is Never Compelling (DarkHorse Livestream #37, aired on August 8, 2020.) Includes discussion of research that finds that far UV light de-activates coronaviruses, and hyper-novelty and the risks of being inside all the time.

• Stop Disrupting Childhood (DarkHorse Livestream #75, aired on April 10, 2021.) Includes discussion of research that suggests that sunlight inactivates SARS-CoV2 beyond what is expected from UV-B alone, suggesting additional health benefits from sunlight.

• Out of Controls (DarkHorse Livestream #126, aired on May 7, 2022.) Includes discussion of vitamin D, NIR, circulating vs subcellular melatonin, the decrease in sunlight exposure in Westerners since the 1800’s, and the values and virtues of sunlight in general.

Pick up a copy of A Hunter-Gatherer’s Guide to the 21^st Century: Evolution and The Challenges of Modern Life for more thinking like this, how moderns have succumbed to reductionist and a-evolutionary thinking at our peril, and how to develop an evolutionary tool kit with which to navigate the world.

1. While most mammals are incapable of seeing into the UV, a few species of rodents have re-evolved the ability. Here’s a nice lay summary of UV vision, and implications for sexual selection, focusing on birds, with nods to some other animals: Withgott 2000. Taking a Bird's-Eye View…in the UV: Recent studies reveal a surprising new picture of how birds see the world. Bioscience, 50(10): 854-859.

2. Fully 70% of photons hitting the body in natural sunlight are NIR photons. Source: Page 142 of Zimmerman and Reiter 2019. Melatonin and the optics of the human body. Melatonin Research, 2(1): 138-160.

3. The larger you are—specifically, the fatter you are—the lower the percentage of your cells that are reached by NIR photons.

4. From page 144 of (again): Zimmerman and Reiter 2019.

5. Several studies are cited to this effect on page 11 (=p100) of Tan et al 2023. Melatonin: Both a Messenger of Darkness and a Participant in the Cellular Actions of Non-Visible Solar Radiation of Near Infrared Light. Biology, 12(1): 89-111.

6. This is one of the hypotheses of Zimmerman and Reiter 2019.

7. This area of inquiry is relatively new, and much is yet to be discovered. Some things that we currently think are true will no doubt turn out to be false. Tan et al 2023 provides a very good recent review.

8. See page 2 (=p90) of Tan et al 2023; and page 156 of Zimmerman & Reiter 2019.

9. As cited on page 15 (=p104) of Tan et al 2023.

10. A partial list of the mechanisms by which melatonin improves mitochondrial function can be found on page 6 (=p94) of Tan et al 2023.

11. Modern “full spectrum” light bulbs are only “full” relative to the visible spectrum. They have no NIR. There is no technical reason for this—we could have LED bulbs that produce photons in the NIR as well. Reductionist thinking got us here. Correctly observing that “light is how we see,” the scientists, engineers and product developers who produced modern LEDs concluded incorrectly that light is only about seeing.

12. Brandeis wrote this in his essay What Publicity Can Do, which was included in his 1914 book Other People’s Money and How The Bankers Use It. The full quotation is “Publicity is justly commended as a remedy for social and industrial diseases. Sunlight is said to be the best of disinfectants; electric light the most efficient policeman.” Brandeis was borrowing from an 1888 book by James Bryce, quoted here.