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

When you think of the sun, how does it make you feel? Most people instinctively respond: warm, bright, happy. The comfort of sunlight on your skin is almost a primal sense of pleasure. Yet behind this seemingly simple intuition lies a complex set of mechanisms that science still doesn't fully understand.

In a conversation about the sun, a 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 influence 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 feel happy—this is nearly a universal human experience. 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 cascade of complex biochemical reactions.

In fact, solar radiation isn't a single beam of light but a continuous electromagnetic spectrum spanning ultraviolet (UV), visible light, and infrared radiation. 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, blue light in the visible spectrum (around 480 nm) is critical for circadian rhythm regulation, and red and near-infrared light are believed to have potential cell-repair properties. It's this diversity of wavelengths that makes sunlight's effects on the body far more complex and multidimensional than what we perceive in daily life.

As the book reveals, the sun has "all sorts of mysterious effects" on the human body, many of which remain scientific enigmas. The book's central argument is also quite subversive: in an era that universally teaches people to "wear sunscreen and avoid the sun," 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 far more extraordinary than we typically give it credit for. Consider this: photons travel from 93 million miles (approximately 150 million kilometers) away, crossing the vast expanse of space to finally reach our bodies, where they trigger a series of biological reactions.

How Photons Interact with the Human Body

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

To fully appreciate this "magic," we need to return to the photon's point of origin. 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 cycles of absorption and re-emission in a process called a "random walk," taking an average of 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 cross the 150 million kilometers of vacuum at the speed of light (approximately 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 the skin, eyes, and even molecules inside our bodies, participating in the regulation of our circadian rhythms, hormone secretion, vitamin D synthesis, and other critical systems.

In other words, what we experience when we bask in sunlight each day 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 universally emphasized the importance of sun protection—applying sunscreen, avoiding intense sun exposure, and reducing UV contact. These recommendations do have scientific backing, particularly in preventing skin cancer. However, In Defense of the Sun attempts to offer a more balanced perspective.

The modern sun-protection movement began in the 1980s when scientists discovered that ozone layer depletion was increasing surface UV radiation, and rising skin cancer rates triggered widespread public health concern. Active ingredients in sunscreen fall into two categories: 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). However, in recent years, some chemical sunscreen ingredients (especially oxybenzone) have been found to penetrate the skin and enter the bloodstream, raising concerns about potential endocrine disruption. 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, with 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:

• Vitamin D Synthesis: Ultraviolet B (UVB) rays in sunlight are the key trigger for the body's vitamin D production. This synthesis 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 to pre-vitamin D3. Subsequently, under the influence of body heat, pre-vitamin 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 underscores the degree to which modern humans have become "estranged" from sunlight.

• Mood Regulation: Sunlight stimulates serotonin secretion, helping improve mood and alleviate depression. Serotonin (5-hydroxytryptamine) is a neurotransmitter that plays a key 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) in the retina 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 depressive subtype that frequently flares during the reduced daylight of winter—is a prime 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 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—who discovered the molecular mechanisms controlling circadian rhythms. Natural light is the strongest external signal (called a "zeitgeber") for calibrating this internal clock; blue light components in morning light transmit signals through specialized photoreceptor cells containing melanopsin, suppressing melatonin secretion and activating wakefulness signals. Modern humans spend extended periods under artificial lighting, and circadian rhythm disruption has become a widespread issue 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 strikes 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 lowered 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 protection from moderate sun exposure may, at the public health level, outweigh its skin cancer risk.

• 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 avoidance of sunlight may carry overlooked health costs of its own.

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

Conclusion: Sunlight Deserves a Fresh Look

The sun—that everyday presence we take for granted—actually holds scientific wonders far beyond our routine understanding. From the physical journey of photons born in the sun's core, wandering randomly for over a hundred thousand years to reach the solar surface, then crossing space in 8 minutes and 20 seconds to arrive on Earth, to their interaction 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 the next time you step outside and feel that warmth, you might pause for a moment and reflect: this simple pleasure is actually a miracle spanning 150 million kilometers. And when it comes to the complex relationship between the sun and human health, science still has much work to do. Finding that proper balance between sun protection and sun exposure may be the true health wisdom we need.