The Effects of Sunlight on the Human Body: Why We Can't Live Without the Sun

Sunlight Makes Us Happy: A Seemingly Simple Question Full of Mysteries

What do you feel when you think of the sun? Most people instinctively respond: warmth, brightness, happiness. The comfort of sunlight on your skin is almost a primal pleasure response. Yet behind this seemingly simple intuition lies complex mechanisms that science has yet to fully understand.

In a conversation about the sun, the host admitted that only after reading a book specifically exploring sunlight and its effects on the human body did they realize how much more mysterious the sun's impact on us truly is. The book is called In Defense of the Sun, by Rowan Jacobson.

Sunlight Brings More Than Just a Good Mood

Sunlight makes us happy — this is nearly a universal human consensus. But this pleasure isn't merely a psychological illusion — it has profound physiological underpinnings. Multiple wavelengths within sunlight interact with our bodies in different ways, triggering a series of complex biochemical reactions.

In fact, solar radiation isn't a single beam of light but a continuous electromagnetic spectrum spanning from ultraviolet (UV), through visible light, to infrared. Ultraviolet light is further divided into UVA (320–400 nm), UVB (280–320 nm), and UVC (100–280 nm), with UVC almost entirely absorbed by the ozone layer. Different wavelengths produce distinctly different biological effects: UVB triggers vitamin D synthesis but can also cause sunburn, UVA penetrates deeper skin layers, the blue light band (~480 nm) within visible light is crucial for circadian rhythm regulation, and red and near-infrared light are believed to have potential cell-repair-promoting properties. It's precisely this wavelength diversity that makes sunlight's effects on the body far more complex and multidimensional than we typically perceive.

As the book reveals, the sun has "all kinds of mysterious effects" on the human body, many of which remain scientific enigmas to this day. The book's core argument is also quite subversive: in an era that universally teaches people to "wear sunscreen and avoid light," the author attempts to re-examine sunlight's positive health value.

Understanding Sunlight's Effects from a Physics Perspective

From a purely physical standpoint, sunlight is actually far more "extraordinary" than we typically give it credit for. Consider this: these are photons from 93 million miles (approximately 150 million kilometers) away, traversing the vastness of space before finally reaching our bodies and triggering a cascade of biological reactions.

How Photons Interact with the Human Body

These photons emitted from a distant star arrive at Earth and somehow "make us feel happy when we step outside." It sounds almost magical, but it's a real physiological phenomenon.

To understand the full picture of this "magic," we need to return to where photons are born. The sun's core fuses hydrogen atoms into helium through nuclear fusion reactions (primarily the proton-proton chain reaction), releasing enormous energy. This energy is produced in the form of photons, but a photon's journey from the sun's core to its surface is extraordinarily long — due to the high density of plasma inside the sun, photons undergo countless absorptions and re-emissions in a process called a "random walk," averaging roughly 100,000 to 170,000 years. However, once a photon leaves the sun's surface, it takes only about 8 minutes and 20 seconds to traverse the 150 million kilometers of vacuum at the speed of light (~300,000 km/s) to reach Earth. In other words, the energy "seed" of the sunlight shining on you right now may have been born in the sun's core over a hundred thousand years ago.

Photons interact with our skin, eyes, and even molecules inside our bodies, participating in the regulation of our circadian rhythms, hormone secretion, vitamin D synthesis, and many other critical systems.

In other words, what we experience daily while basking in sunlight is actually an energy transfer and biochemical reaction spanning 150 million kilometers. And this is merely the tip of the iceberg when it comes to the sun's many effects.

Re-examining the Health Balance Between Sun Protection and Sun Exposure

In recent decades, mainstream health advice has overwhelmingly emphasized the importance of sun protection — applying sunscreen, avoiding excessive sun exposure, and reducing ultraviolet contact. These recommendations certainly have scientific merit, especially for skin cancer prevention. However, In Defense of the Sun attempts to present a more balanced perspective.

The modern sunscreen movement began in the 1980s when scientists discovered that ozone layer depletion was increasing surface UV radiation, and rising skin cancer rates sparked widespread public health concern. Active ingredients in sunscreen are divided into physical sunscreens (such as zinc oxide and titanium dioxide, which work by reflecting UV rays) and chemical sunscreens (such as oxybenzone and avobenzone, which absorb UV rays and convert them to heat). In recent years, however, some chemical sunscreen ingredients (especially oxybenzone) have been found to penetrate the skin and enter the bloodstream, and their potential as endocrine disruptors has sparked controversy. Additionally, some epidemiological studies have produced seemingly contradictory findings: for example, a study tracking nearly 30,000 Swedish women over 20 years (published in the Journal of Internal Medicine) found that women who completely avoided sunlight actually had higher all-cause mortality than those with moderate sun exposure — an effect size comparable to the risk of smoking. Such findings form the scientific foundation upon which books like In Defense of the Sun build their arguments.

Health Benefits of Moderate Sun Exposure

Author Rowan Jacobson draws on extensive research to explore the many potential benefits of moderate sun exposure for the human body:

• Vitamin D Synthesis: Ultraviolet B (UVB) rays in sunlight are the key trigger for the body's vitamin D production. This synthesis process is a precise, multi-step biochemical reaction: when UVB photons strike 7-dehydrocholesterol (a cholesterol precursor) in the skin's epidermal layer, the photon's energy breaks specific chemical bonds in the molecule, converting it into previtamin D3. Under body heat, previtamin D3 slowly isomerizes into vitamin D3 (cholecalciferol), which then undergoes two hydroxylation steps in the liver and kidneys to become biologically active calcitriol — a hormone involved in regulating calcium-phosphorus metabolism, immune function, gene expression, and hundreds of other physiological processes. An estimated 1 billion people worldwide have insufficient or deficient vitamin D levels — a fact that indirectly illustrates the degree of modern humanity's "estrangement" from sunlight.

• Mood Regulation: Sunlight stimulates serotonin secretion, helping improve mood and alleviate depression. Serotonin (5-hydroxytryptamine) is a neurotransmitter that plays a crucial role in mood regulation, appetite, sleep, and cognitive function. Sunlight influences serotonin levels through at least two pathways: first, light signals transmitted through intrinsically photosensitive retinal ganglion cells (ipRGCs) reach the brain's raphe nuclei, directly stimulating serotonin synthesis and release; second, recent research has discovered that the skin itself possesses an independent serotonin synthesis system, and UVA exposure can affect systemic circulation. Seasonal Affective Disorder (SAD) — a subtype of depression that frequently occurs when winter daylight decreases — is a classic example of the sunlight-mood connection, affecting approximately 5%–10% of the global population.

• Circadian Rhythm Regulation: Natural light helps calibrate the body's biological clock and improve sleep quality. The human circadian rhythm is an approximately 24-hour biological cycle coordinated by the master clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The 2017 Nobel Prize in Physiology or Medicine was awarded to three scientists — Jeffrey Hall, Michael Rosbash, and Michael Young — for discovering the molecular mechanisms controlling circadian rhythms. Natural light is the strongest external signal (called a "zeitgeber") for calibrating this internal clock. The blue light component in morning light transmits signals through specialized photoreceptor cells containing melanopsin, suppressing melatonin secretion and activating wakefulness signals. Modern humans spend prolonged periods under artificial lighting, and circadian rhythm disruption has become a widespread problem linked to insomnia, metabolic syndrome, cardiovascular disease, and even increased cancer risk.

• Cardiovascular Protection: Beyond the mechanisms above, another important pathway through which sunlight affects health involves nitric oxide (NO). Human skin stores large quantities of nitrites and nitrates. When UVA irradiates the skin, these substances are photolyzed into nitric oxide and released into the bloodstream. Nitric oxide is a potent vasodilator that can lower blood pressure and improve vascular endothelial function. Research by dermatologist Richard Weller's team at the University of Edinburgh found that just 20 minutes of UVA exposure significantly reduced subjects' blood pressure — an effect independent of vitamin D changes. Given that cardiovascular disease is the world's leading cause of death, this finding suggests that the potential cardiovascular protective effects of moderate sun exposure may outweigh its skin cancer risk at the public health level.

• Disease Prevention: Some studies suggest that moderate sun exposure may reduce the risk of certain chronic diseases, including multiple sclerosis, type 1 diabetes, and certain types of internal organ cancers.

Jacobson doesn't deny the value of sun protection; rather, he reminds us that excessive sun avoidance may equally carry overlooked health costs.

This stance of "defending the sun" is essentially a call for a more scientific, more dialectical attitude: we should guard against sunlight's potential harms while fully recognizing and harnessing its positive effects on the body.

Conclusion: Sunlight Deserves Our Renewed Appreciation

The sun — that familiar, everyday presence we take for granted — actually harbors scientific wonders far beyond our ordinary understanding. From the physical journey of photons born in the sun's core, undergoing a random walk of over 100,000 years to reach the solar surface, then crossing space in 8 minutes and 20 seconds to arrive at Earth, to their interactions with the precise molecular machinery in our skin and eyes — triggering vitamin D synthesis, serotonin release, nitric oxide production, and circadian rhythm calibration — sunlight remains a topic that is simultaneously familiar and mysterious.

Perhaps next time you step outside and feel that warmth, you might pause for a moment to consider: this simple pleasure is actually a miracle spanning 150 million kilometers. And regarding the complex relationship between the sun and human health, the scientific community still has much work to do. Finding the right balance between sun protection and sun exposure may be the true health wisdom we need.