We are all made of sunshine. If that sounds fluffy to you, think again! Our connection to sunlight is all science, billions of years in the making, and it’s important to understand if we want to achieve optimal metabolic health and happiness. The functionality of our 37 trillion cells depends on that yellow orb in the sky constantly transporting little packets of energy (photons) to us through space. And our bodies need that energy at the right times and in the right dose to feel our best.
Just as food is molecular information for our bodies, sunlight is energetic information that tells our brains and cells how to function. I didn’t become disciplined about getting sun exposure until I fully recognized the direct physical relationship the sun’s energy has on the structure and action of my body. If you read this and leave with more understanding of your body’s relationship with sunlight, and more motivation to sleep in consistent patterns and get morning direct sun exposure, I’ll consider it a win!
Here are five specific ways that sunlight directly impacts our metabolic health.
1. It Starts With Plants: How Sunlight Creates Glucose
In one of nature’s most incredible feats (which most of us have probably overlooked since middle school), the glucose molecules generated in plants are a direct product of the sun’s energy. Through light-dependent chemical reactions, plants use photons, water, and carbon dioxide to produce simple sugars, which eventually transform into various types of complex carbohydrates like starch that they (and we) use for energy. When we eat those carbohydrates, our bodies break down those molecules to drive the production of energy we can use (ATP). Energy that begins as light winds up in the bonds of the sugars.
Another byproduct of photosynthesis is oxygen, which every cell in our body needs for aerobic metabolism. Without plants on earth providing this crucial gas as a byproduct of their chemical dance with sunlight, we—and almost all other living things—would not survive
2. Photoreceptors: Our Brain’s Window to Sunlight’s Information
Our eyes are the brain’s access port to natural light, and we have photoreceptors (light-sensitive cells) in our retinas that receive, are stimulated by, and react to a particle of sunlight. Photoreceptors have molecules in them that undergo a tiny chemical change when they absorb light [see below], triggering a cascade of events that leads to an electrical impulse sent along axons to the next neuron and the next, all leading to the suprachiasmatic nucleus (SCN) and other parts of the brain.
Upon exposure to light, parts of the SCN, which is in the hypothalamus, are stimulated to kick off genetic, hormonal, and neurotransmitter-mediated processes that affect the activity of the brain and nearly all other cells in the body.
When we are exposed to “irregular photic signals” (think inconsistent light exposure because of erratic sleep or being inside so much that our bodies don’t get exposure to the normal cycles of the sun), this complex cascade becomes distorted and can negatively impact nearly all physiological activities. By getting sunlight first thing in the morning and avoiding excess artificial light near bedtime, we signal to the SCN the time of day and set our bodies up to time genetic and hormonal signals appropriately.
I have personal experience with the health impacts that can arise secondary to erratic sleep and lack of sunlight. For years when I was a surgical resident, I was on-call two to three nights per week, staying inside the hospital for 24- to 30-hour stretches. I would arrive at 5:50 a.m. (in the dark), work a full day in the hospital and operating room, stay overnight for any emergencies, and leave the next day around noon. Then I would drive home, close my blackout shades, and sleep.
Many days went by with near-zero direct sunlight exposure. And what happened? My body broke down: From an initial state of perfect health, I quickly developed acne, chronic neck pain, irritable bowel syndrome, and more belly fat. I was even more emotional than normal. When I ultimately left the surgical world and immediately got back to a more normal sleep schedule and regular sun exposure (along with better quality food and less stress), every symptom “magically” disappeared in about a month. We are unwise to mess with the billions of years of development of the relationship between biological organisms and the sun, and yet modern life makes it exceedingly challenging to be naturally regulated by light.
When I think about the importance of exposing my eyes to direct sunlight in the morning, I am thinking about all those little photons traveling 92 million miles from the sun—hitting my retinal cells, causing tiny structural shifts that lead to nerve cells firing—the signal that my brain receives, and the subsequent information my brain gives to my entire body about hormone levels, genetic expression, and chemical signaling. My lived reality is downstream of the choices I make about when and how much sunlight I expose my brain to.
3. Circadian Rhythm and Metabolic Health
The SCN functions as the body’s master clock or pacemaker. It’s involved in our sleep-wake cycle—also known as our circadian rhythm—which impacts everything from food intake, insulin sensitivity, glucose control, and energy expenditure. Our cells have what are called clock genes, which function to self-regulate their expression on close to a 24-hour cycle, although light (and food) timing affects this pattern.
In rats, getting bright light exposure in the morning leads to lower weight and glucose levels (as well as lower anxiety and depressive behaviors). Most of us are not getting that optimal exposure, however. As one paper states: “current lifestyle and social habits, such as eating or working at night, being exposed to artificial light at night, and altered sleeping schedules are among the factors that can cause circadian disruption.” The result is dire for metabolism. This lack of alignment with natural light cycles contributes to pancreatic dysfunction (the organ that makes insulin), issues with glucose and fat metabolism, and increased risk of cardiovascular disease, cancer, obesity, and Type 2 diabetes.
Our master clock also impacts autonomic function, which controls our involuntary processes—think everything from digestion to a host of nervous system responses. I mentioned in our last newsletter how our thoughts impact how we make and use energy. The SCN regulates the release of cortisol, a main stress hormone. So when our circadian rhythm is inconsistent and dysregulated, it can also significantly impair our stress-response system.
Animal studies show that our stress response is dependent on the time of day because our clock genes interplay with our stress hormones. So although more research is needed, we know that our circadian rhythm and our stress system interact. And dysregulation of this interaction negatively affects metabolism and body weight.
4. Sunshine, Serotonin, and Mood
We also know that sunshine can impact mood, and that mood is interwoven with our metabolic health. In some people, decreases in sun exposure can trigger major depression with seasonal pattern, a condition formerly called seasonal affective disorder (SAD). In other people, mood changes related to decreased sun exposure may be much more subtle.
But less exposure to sunlight is linked to lower serotonin levels. And a correlation exists between higher levels of serotonin and more exposure to natural light. Serotonin modulates mood. More research is needed on how exactly sunshine impacts this neurotransmitter, but natural light appears to promote serotonin-1A receptor binding in the brain. Light could potentially impact serotonin via our skin, as well.
Serotonin also plays a role in metabolic regulation and related disease processes. Increased serotonin signaling appears to help modulate appetite (decreasing it) and improve glucose control.
5. The Sun, Vitamin D, and Metabolic Health
We also have vitamin D receptors in almost every cell. We can get this micronutrient from foods, like trout, mushrooms, and salmon. But we get our majority of the sunshine vitamin—which is really a steroid hormone—from sunlight.
The vitamin D precursor molecule 7-dehydrocholesterol in the skin absorbs the energy of UVB radiation from the sun, which causes a microscopic change in the bonds of the molecule [see below], creating previtamin D3. It then heads to the liver, and ultimately to the kidneys to turn it into its active form. It is remarkable to me how the tiniest jolt of energy from the sun and the tiniest change in shape of a molecule has a profound impact on our mood and metabolism.
Vitamin D has strong impacts on metabolic health, yet many people in the United States and elsewhere are deficient in it. Low vitamin D levels are associated with obesity, insulin resistance, diabetes, and other disease processes. We need more research on the exact mechanisms regarding how vitamin D impacts metabolic health. But the hormone does have an anti-inflammatory effect. Additionally, research shows that it may increase insulin secretion from pancreatic beta cells. While many labs say that anything above 30 ng/mL is normal, many precision-medicine practitioners recommend shooting for much higher, like 40-80 ng/mL.
The Bottom Line
Whether we are eating or just going about our day, sunshine is impacting us at the cellular level—albeit sometimes indirectly. And it’s awe-inspiring how much sunlight affects our core physiology, mood, and metabolic health.
I often think about how much this 4.6-billion-year-old star impacts our physiological processes, transferring its energetic information to our bodies over nearly 100 million miles. And it’s a good reminder for me to expose myself to some natural light every day to keep these primordial cellular clocks synced. I stack the habits of brushing my teeth with stepping outside to get some sunlight so that I make sure I’m getting at least 2 minutes of sunshine first thing. Remember, window glass blocks a significant amount of light, according to Stanford neurobiologist Andrew Huberman, PhD, so try to actually get outdoors. And don’t worry about clouds; the sun’s energy shines right through.