Sun Cult, Part 2: The Rhythms of Night and Day

How circadian biology secretly rules our lives

In last week’s blog post, I talked about how the seasonal light changes affect our mood and physiology, as well as the life rhythm of every other living being around us. Today, I will discuss another important cycle: the alternating pattern of day and night.  

In the five years of making a weekly pilgrimage to my favorite rustic Florida Gulf Coast beach, I noticed a curious pattern. When seagulls are not preoccupied by stealing Cheetos out of tourists’ beach bags, they like to simply stand in a pack facing the same direction. The strange part is that they are always facing the sun, staring directly into it—and sometimes with their mouths open, as if waiting for a shiny killifish to just fly into it. They do this for hours: just standing there sungazing, particularly in the wintertime.  

Seagulls are not alone in this behavior.  All of nature observes the sun cult. Sunflowers are perhaps the most famous for tracking its daily motion, but so do many other plants, like traveler palms that rotate their gorgeous fans to catch the sun’s rays. Reptiles bask in the sun to warm up their cold blood. Insects use the morning light to shake off the night’s torpor. Monarch butterflies make their annual journey southwest guided by the sun’s compas. Mammals are the most active during daylight hours, unless they are nocturnal, in which case the activity pattern is reversed.  

Plants exclusively work during the day: they make their own food by photosynthesis, converting light energy and carbon dioxide into nutrients. At night, they switch the process and start burning sugar for fuel, just like we do. It is not surprising then that plants open and close their leaves and flowers to get the maximum amount of sunlight. The crazy thing is that even if you put a plant in a dark room, it will continue opening and closing its leaves in the usual time, as if it has a built-in clock for doing that. 

Life on Earth is inextricably linked to the rhythms of day and night.  

The underlying mechanism that sets those rhythms in motion is called circadian biology. The scientists who showed the molecular connection between light and body’s innate knowledge of the time were recognized by the 2017 Nobel Prize in Physiology. Since (and before) then, researchers have shown that circadian biology is linked to almost every process in the body, and that ignoring this fundamental principle of biology can have grave consequences.  

The intricacies of light 

Some researchers simplify circadian biology down to the cycles of light and darkness. But it’s way more complex than that. In my previous blog, I described how the sunlight spectrum changes throughout the year. The same spectral shifts occur every day and affect us on, I would say, an even more profound level. And it all has to do with the daily arc the sun traces in the sky. 

When the sun is higher up in sky during the summer months, it hits the Earth at a close to a 90-degree angle. At this angle, more UV and blue light portions of the spectrum pass through the atmosphere. The result is a more intense, brighter light environment that is heavier on the short-wave, high intensity light. In the fall and winter, the maximum angle the sun reaches is closer to 40-50 degrees (depending on your latitude), which means that the UV light intensity is much lower—so much so, that it is not even enough for your skin to synthesize vitamin D.  

The angle of the sun is the same reason why the spectral ‘temperature’ of light changes throughout the day. In the mornings, we get more of the red, orange, and yellow wavelengths. The gorgeous crimson sunrise gradually shifts into a soft yellowish and then pure white, which, of course, still contains all of the colors of the light spectrum. The proportion of the different color wavelengths changes throughout the day, however. The natural sunlight has the highest amount of blue light around noon, after which its proportion begins to decrease and shift towards the yellows and reds again.  

So, how does this all tie into circadian biology? The circadian clock is set by the light—and not just by the length of the day, but by the different colors of the light spectrum. The red morning light activates certain receptors in our eyes and skin, while the blue noon light shuts those off and activates different ones. These receptors pass on signals to the rest of the body, telling it to produce hormones like cortisol, which regulate our physiology.  

The circadian clock is the internal time keeping system that governs virtually EVERY process in our bodies. It tells us when it’s time to go to sleep, when it’s time to wake up, when it’s time to be hungry, when to be the most physically or mentally active, when to have the highest blood pressure, lowest body temperature, more of one hormone and less of the other, controls your blood sugar and so many other things. It is the master regulator conducting the beautiful symphony of dynamic and adaptable living organisms.  

What happens when things go wrong 

All of us are probably familiar with what happens when there is a mismatch between our internal biological clock and external environment—like when you fly across the world and all of a sudden, your entire life rhythm is flipped on its head. It takes many days to adjust to the new time zone, and those days (and nights) are not fun. However, synchronizing your internal clock to the one on the wall will be much easier if you simply go outside and bask in the natural light to accelerate the adaptation process.  

Jet lag is annoying, but we rarely have to deal with it. What is a lot more common, however, is chronic, disruption in circadian rhythms. Scientists have identified links between chronic circadian rhythm disruptions and diseases like diabetes, cancer, as well as obesity. Night-shift nurses, for example, show statistically higher incidence of cancer. Even mild exposure to light at night can mess up the melatonin levels, increasing the risk of breast cancer in women.  

These effects have to do with general mis-regulation of circadian biology and, specifically, with exposure to blue light. While blue light is always present in the visible spectrum of sunlight, indoor LED lighting has a much higher proportion of blue and much less of the red light than does natural light. Similarly, our computer, phone and TV screens are all lit up with the brightest blue. And unlike the sun, the amount of indoor blue light does not diminish at the day grows old. 

Why is that a big deal? This is where it starts getting interesting: the high amount of blue light our eyes receive at the peak of the day signals the body that it’s time to be active and productive. Carpe diem, it tells your brain, your hormones, and really, every cell in your body. This elevates your blood glucose, cortisol and ghrelin (the hunger hormone), causing insulin resistance over time. It basically keeps your body in ready-to-go stress mode, depriving it of much-needed chillout time.  

Blue light also shuts off melatonin production. Melatonin is the hormone that is not only responsible for a good night’s rest, but also the function of the immune system. Many repair processes, like autophagy happen at night, and my messing up that important physiological process, we compromise our overall health. And before you ask: Taking the pill is not the answer. An artificial melatonin spike from the pill or patch can actually worsen the symptoms of circadian disruption.  

Resetting our clocks

When you are on vacation, you will notice how much better you sleep and feel overall. It’s not just the lack of stress. It’s also from spending more time outside, in the natural light that synchs up your circadian clock. By letting our body witness the transition from the soft light of dawn to the brilliance of noon and the golden splendor of sunset that recedes into dusk, it knows exactly what time it is and what it is supposed to do.

Few people know this, but melatonin biosynthesis in our body starts in the morning when the light signals to your body that it’s time to get up. The bright sun at noon shifts hormone production and redirects the resources in other processes. We synthesize vitamin D, for example, which is responsible for regulating the immune system. Then, after the day’s work is done and the earth is shrouded in darkness, melatonin is released by the pineal gland, prompting well-deserved rest. 

But of course, in this crazy age, how many of us are able to afford the luxury of spending time outside and going to bed at a reasonable hour? No wonder sleep and mood disruptions, stress, diabetes and other chronic diseases are on the rise.

Animals and plants are all governed by the sun’s daily patterns. Why should we be different or exempt from it? Yes, we can “hack” our biology with artificial light, indoor heating, and stimulants to improve our productivity. But is it healthy? Is it sustainable? And is it how we are meant to live our lives on planet Earth?