The Power of the Circadian Clock and the Sun

Imagine a time period tens of thousands of years ago when our hunter-gatherer ancestors dwelled upon the earth’s surface. It may be resemblant of a time when the daily activity of early, modern humans was almost solely determined by the power of the sun and its cosmic rays, shedding down upon populations before surely setting for the evening. Daylight was expensive and had to be used wisely in order to find that day’s meal necessary for these tribes to thrive. As the sun began to rise and the birds began to chirp, these sensory cues were taken into the brain via the eyes, skin and ears and molded the rhythm of the day—not knowing that the beginning of their day, greeted by light, would be a determinant as to when the eyes would soon close, allowing for the sleep-regenerative properties to reoccur and continue to cycle. Human beings are a creature of this earth, and you better believe we were meant to be under the sun.

Fast forward to the present day, not only do we, on earth, continue to spin about our axis and take cosmic trips around the sun, but the lights never really seem to turn off. Although the invention of artificial, electrical lighting was a necessary advancement that allowed the human race to soar to new heights that were little-thought to be obtainable, there is much evidence we overuse this new-found light, in many forms; especially at inappropriate times. 10,000 years ago may seem like an unfathomable amount of time to you and I; however, it is rather quite minuscule when referencing the evolutionary timescale. As our social culture evolves at an insanely rapid, exponential rate, our biology and the bodily systems present today are mostly unchanged. In a way we are simply cavemen in business suits. Both our human physiology then and our human physiology now are still regulated by the exact same processes; in this regard, we will be referring to the circadian clock.

 What is the circadian clock and the circadian rhythm?

The circadian clock is the master regulator of our body, and it can receive and respond to many inputs. But, most importantly, it responds and calibrates the rest of the body by taking in light. This master clock lives within the center of our brains, in a region called the suprachiasmatic nucleus (SCN). The SCN works in a rhythmic fashion, where certain proteins are produced, then inhibited, produced, then inhibited, and so on. This activation and inhibition relationship occurs in a clock-like manner, lasting slightly over 24 hours [1] . If the SCN is the master clock, that may infer that there are other clocks too, right? In fact, almost all tissues and thus, bodily systems within the human body have their own circadian clock, which may be more or less active during certain time periods throughout the 24-hour cycle, based on the information provided from the master—SCN—clock. The circadian clock has a direct influence on not only your sleep, but also your metabolism, hormone production, mood and inflammation response [1]. There are many ways in which our circadian rhythms can get (for lack of a better term) out of wack. Especially in today’s day and age. Some of the main culprits that are out to skew your circadian rhythms are the lack of proper sunlight, exercising (or lack of), over eating—especially at the wrong times, and for too long—and of course the over-exposure to artificial light. Here, we will delve into the dangers of living a life that’s out of rhythm compared to what our biology intended for us, some of the ways to begin to fix our broken clocks, some of the misconceptions regarding the sun, as well as some of the many benefits the sun can provide to us—at no cost!

The dangers of a broken clock:

The Solar Radiation Spectrum:

First let’s examine the visible light and the solar light radiation spectrum, which can be observed in Figure 1.  

These are the different wavelengths of light in which are emitted by the sun, ranging from more intense shorter wavelengths of ultraviolet (UV) rays, to the visible light spectrum, all the way to the longer, less-intense infrared (IR) rays.

Phase shifts—Advances and Delays:

Some terminology you should be familiar with is the idea of circadian phase advances and phase delays. I will be referring to these shifts throughout the article. A phase delay is essentially a forward shift in your circadian rhythm due to suppression of melatonin production; this can occur for a number of reasons, whether it’s staying up later than usual on a weekend (“social jet lag”), actually flying over time-zones, the use of stimulants, and even a poorly timed meal or exercise session. A phase advance is when the onset of melatonin production occurs earlier in the evening relative to previous nights, and this can also occur via many factors, such as early morning exposure to light, decreasing light at night, or even the use of some pharmaceuticals. 


In an ideal scenario, as the sun begins to set and the lights go out—besides maybe a small social gathering around a fire—your pineal gland gets information from the SCN to begin the production of a hormone called melatonin. Our photoreceptors (the cells that detect light) in our retina of the eyes will become less-stimulated, in that there isn’t much light coming in since the sun began to set, and will allow for the production of  “dim light melatonin onset” (DLMO). This is really just the first time in the evening—about 2-3 hours before habitual bedtime—when melatonin production begins to occur [1].  When light is still coming in and being detected by the photoreceptors in the retina, then melatonin production is inhibited. There are several different photoreceptors within the eyes and the skin, but the most important photoreceptor regarding your circadian clock is one called ipRGC. This ipRGC photoreceptor is especially sensitive to blue or short-wavelength light (~480 nm) [1]. Blue light is of high concentration coming from sources of artificial light.

Knowing this, there is plenty of data to suggest that exposure to blue light in the evening hours can activate the ipRGC photoreceptor and thus suppress melatonin production [1, 3, 6, 8, 10]. The “light-at-night” hypothesis is a well-studied and growing body of research in which there are many implications, dangers and concerns regarding suppressed melatonin production. Not only is melatonin an important regulator of circadian rhythms and sleep, but it also has proven to be a sufficient antioxidant and anti-carcinogenic agent. Melatonin suppression has been linked to melanoma skin cancer as well as breast cancer [3]. Melatonin, which is also produced in the skin (since we have photoreceptors there as well) is able to exhibit growth-suppressive effects in which the melatonin will aid to combat damage in potentially provoked by UV rays and thus mitigate the possibility of melanoma tumor growth [3]. There are studies showing that pilots, and flight attendants have an increased risk of melanoma cancer growth due to the suppression of melatonin production when traveling over multiple time zones and disrupting the natural circadian rhythmicity [3]. This same risk may also be of concern to frequent travelers and night-shift workers. In addition, the circulation of melatonin has been shown to reduce circulating levels of the hormone estradiol, so when melatonin is inhibited due to increased artificial light at night, it is hypothesized that there is an increased risk for hormone-dependent cancers such as breast cancer [3, 6]. 

Metabolism, Mitochondria and Fat Regarding Circadian Biology:

Not only can circadian dysfunction result in abnormal sleep patterns, increased risks for cancer and other chronic diseases, but it can have great impact on metabolic processes. In terms of the homeostatic processes that are required for energy balance and utilization, it has been determined that many of the active metabolic tissues also operate in a clock-like fashion that is ultimately regulated by the SCN [7]. Adipose tissue (fat stored in the body) is an energy dense tissue that is capable of producing high amounts of ATP (the energy currency for the body) if it is able to be utilized properly. Some of the clock-like genes that are found in adipose tissue, were purposely mutated in a lab experiment involving mice—therefore these rhythmic genes were not longer able to be expressed. The results of this experiment interestingly showed that the mice with the mutant clock genes were not utilizing their own body-fat stores (lipolysis) properly, were gaining more fat tissue and were at increased sensitivity to fasting [7]. Basically, the mice with out of wack circadian rhythms were becoming obese, experiencing reduced energy expenditure, as well as having altered food-intake behavior. This suggests that the adipose-tissue clock is essential for rhythmic fatty acid release into the circulation, which then entrains circadian, rhythmic feeding behavior [7]!

It was mentioned that light is the main sensory input that is provided to develop the behavior of the master clock. However, it can also be heavily impacted by food intake: thus, the sleep/wake as well as the feeding/fasting cycles are of vital importance to develop healthy circadian rhythms. 

Another interesting mice experiment involved two groups of mice that were fed a high-fat diet; group one had access to the food whenever they wanted (ad libitum), as where group two had a feeding window that was restricted to their normal circadian basis (time-restricted eating). The findings were that the time-restricted group displayed increased thermogenesis—which is the production of heat, thus burning calories— reduced fat-mass and, improved nutrient utilization [7]. Consuming food in a rhythmic fashion is not only one of the master clock inputs, but it also profoundly affects the mitochondria’s ability to metabolize and utilize nutrients.

The Power of the Sun:

As mentioned earlier, the sun emits a various range of solar radiation. Sun exposer has gotten a bad wrap over the last several decades, especially the UV rays, which have been shown to cause skin cancer, aging, and immune suppression [12]. However, there are mechanisms in place to assist in harnessing the benefits of the sun and mitigating the risks. When the sun begins to rise every morning, it is lower in the sky and the shorter wavelengths such as the UV rays are at a minimum, however, the longer IR rays are at an increased ratio. The IR wavelengths are able to actually penetrate through the skin and—at increased wavelengths—can penetrate into deeper tissues and exhibit healing effects. At reasonable doses, this low-level light can help to facilitate processes that can result in wound healing and modulation of chronic inflammation [2, 5]. There is even evidence to suggest that these early morning IR waves can help precondition the skin in order to prepare for the mid-day sun—which is high in UVB rays: the ones that can cause a sunburn and make your skin tan [12]. From an evolutionary perspective, this makes complete sense, since our ancestors—being awoken by the sunrise in a rhythmic fashion—would be exposed to high amounts of low-level IR light; as the sun became higher in the sky—increasing the UVB emission—their skin may have been more prepared to handle those waves. This process is called photoprevention [12]. As the sun began to set, also becoming lower in the sky and thus emitting more low-level IR rays, then the skin is able to take advantage of the healing processes of the sun in the evening, combating any potential damage that may have occurred during the day via photo repair [12]

When examining the UV spectrum, we need to only consider UVA and UVB, both of which are capable of causing tanning and burning, however UVB does so more rapidly. UVA rays will actually penetrate the skin deeper, which could be a more important determinant when regarding photoaging, skin cancers and wrinkling of the skin [11]. An interesting hypothesis was proposed that suggested that day-time shift workers showed increased rates of melanoma cancer due to the fact that they worked at indoor facilities containing windows. This is of importance because UVA rays are able to penetrate through the window glass as where UVB rays are unable. UVB rays are  the primary producers of vitamin D. The researchers hypothesized that the UVA rays are able to cause mutations in the skin cells, which then can cause the breakdown of vitamin D that was formed form outdoor UVB exposure; in addition, they suggested the higher cancer rates could also be due to simply low-levels of vitamin D. Vitamin D is of important regard to melanoma cells, because the melanoma cancerous cells can convert the vitamin D into a similar hormone called calcitrol, that will result in the growth inhibition of the cancerous cells as well as triggering the cancerous cells to commit suicide (apoptosis) [11]. Vitamin D impacts every cell in your body and is a very potent fighter of cancer. Outdoor workers have been shown to have lower rates of melanoma cancer compared to indoor workers. Therefore, chronic sun exposure, may exhibit numerous and natural benefits to fight cancer [11]. 

We all know that we are able to supplement with vitamin D; however, is it the same as the vitamin D you obtain from the sun? Not quite, the oral vitamin D you can purchase from a store, is not water soluble and needs a vehicle of travel: LDL cholesterol is needed to transport the oral vitamin D to other parts of your body. LDL is referred to as the “bad cholesterol”. In contrast, the vitamin D you produce via the sun’s rays has a sulfate group attached to it, making it water soluble and thus, it is free to travel through the blood to other tissues and organs [11]. 

There is also compelling evidence that the sun literally can make us happier! That’s right, sun exposure has been linked to increasing natural production of serotonin [10]. Briefly, serotonin is a neurotransmitter that is sometimes referred to as the happy chemical. In addition to increased serotonin levels, there is also evidence that sun exposure has been linked to increased dopamine receptor availability [9]. This is why the sunshine can help to motivate and promote a positive mood. 

How to Fix a Broken Circadian Clock:

For the majority of people living in the modern world—who are constantly bombarded with artificial light, have access to food 24/7 and spend way too much time indoors—they will want to work towards phase advancements and thus, producing melatonin earlier in the evening and allowing for melatonin offset to occur slightly before awakening. One of the most profound ways to generate a phase advance for your circadian clock is to expose yourself to early morning sunlight, and as much as possible, showing as much skin as possible. Remember, the highest concentration of photoreceptors is in the retina, but we also have photoreceptors all across our skin’s surface. A study was preformed where a group of participants were examined for 1 week in a typical modern scenario, and the following week in a natural light setting to determine the effects of natural light on the circadian clock. The artificial light trial had access to, essentially, the modern world where they had all their electronics, artificial light and indoor comfort. During the second week of the natural light trial, the participants were camping in the mountains of Colorado. They did not have access to artificial light, electronics and other typical modern practices. The week of natural light exposure, not only did the participants on average get 4 hours more of natural sunlight, but they also experienced significantly more light within the first 2 hours of waking up. The results also revealed that the participants experienced a roughly 2-hour phase advance in their circadian melatonin production, and the melatonin offset occurred more than 50 minutes prior to waking up [8]. This suggests that even one week of exposure to natural sunlight and diminishing light at night could synchronize your circadian clock with the natural light/dark cycle. This would allow for your groggy low-point of the day to occur just before bed time, and you would be able to wake up feeling refreshed and not tired since the melatonin circulation would have already diminished before awakening [4, 8]. 

Realigning your circadian behaviors may result in not only improved sleep patterns but also may allow you to reach deeper, more restorative phases of sleep; in addition to improved cognitive functioning, motor capabilities, and mood [4]. Another point to consider is that your feeding rhythms may also improve, thus allowing for more efficient metabolism and potentially assisting your mitochondria to burn more of your own body fat.  

For many people this can be a tough task to fully get your circadian clock balanced, so there are some simple fixes that can help push you in the right direction. One would be to wear blue-light blocking lenses in the evening and to minimize the use of electronic devices so that the onset of melatonin production can occur earlier. Some other practices should include earlier-morning sunlight exposure, and increased sun exposer in general; as well as shifting your feeding window forward so that you aren’t eating too close to bedtime—at least 3 hours. Consistent afternoon exercise has also been shown to affect the master clock as well as to increase cognitive abilities [4]. 

 If you’ve made it this far then you have the tools necessary to begin the repair of your biological circadian clock. There is a lot of information present and it doesn’t begin to scratch the surface of the complexity of the human physiology regarding circadian behavior. However, there is enough detail to allow you to make improved choices and work towards better sleep, increased metabolism, and superior biomarkers and rhythms. I hope you all enjoyed and I would love to hear any feedback, questions or comments. You’ve just been MaxifyD!

Thank you!


[1] Emens, J. S., & Burgess, H. J. (2015). Effect of Light and Melatonin and Other Melatonin Receptor Agonists on Human Circadian Physiology. Sleep Medicine Clinics, 10, 435–453.

[2] Tafur, J., & Mills, P. J. (2008). Low-Intensity Light Therapy: Exploring the Role of Redox Mechanisms. PHOTOMEDICINE AND LASER SURGERY, (4), 323. Retrieved from,uid&db=edsbl&AN=RN235594431&site=eds-live 

[3] Kvaskoff, M., & Weinstein, P. (2010). Are some melanomas caused by artificial light? Medical Hypotheses, 75, 305–311.

[4] Schroeder, A. M., & Colwell, C. S. (2013). Opinion: How to fix a broken clock. Trends in Pharmacological Sciences, 34, 605–619. 

[5] Avci, P., Gupta, A., Sadasivam, M., Vecchio, D., Pam, Z., Pam, N., & Hamblin, M. R. (n.d.). Low-Level Laser (Light) Therapy (LLLT) in Skin: Stimulating, Healing, Restoring. SEMINARS IN CUTANEOUS MEDICINE AND SURGERY, 32(1), 41–52. Retrieved from,uid&db=edswsc&AN=000317529400008&site=eds-live 

[6] Brennan, R., Jan, J. E., & Lyons, C. J. (2007). Light, dark, and melatonin: emerging evidence for the importance of melatonin in ocular physiology. EYE -LONDON- OPHTHALMOLOGICAL SOCIETY OF THE UNITED KINGDOM THEN ROYAL COLLEGE OF OPHTHALMOLOGISTS-, (7), 901. Retrieved from,uid&db=edsbl&AN=RN211316401&site=eds-live 

[7] Onder, Y., & Green, C. B. (2018). Review article: Rhythms of metabolism in adipose tissue and mitochondria. Neurobiology of Sleep and Circadian Rhythms, 4, 57–63.

[8] Wright, K. P., McHill, A. W., Birks, B. R., Griffin, B. R., Rusterholz, T., & Chinoy, E. D. (2013). Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle. Current Biology, (16), 1554.

[9] Tsai, H.-Y., Chen, K. C., Yang, Y. K., Chen, P. S., Yeh, T. L., Chiu, N. T., & Lee, I. H. (2011). Sunshine-exposure variation of human striatal dopamine D2/D3 receptor availability in healthy volunteers. Progress in Neuropsychopharmacology & Biological Psychiatry, 35, 107–110. 

[10] Sauermann Kirsten, Bechara Falk G, Vojvodic Mirjana, Bader Armin, Gambichler Thilo, Altmeyer Peter, & Hoffmann Klaus. (2002). Impact of UVA exposure on psychological parameters and circulating serotonin and melatonin. BMC Dermatology, (1), 6.
[11] Mercola, Joseph. “Increase in Melanoma Skin Cancer NOT Caused by Sun Exposure.”,

[12] “Infrared Does More Good than Bad for the Skin: How Can We Learn from the Sun.” Atlas of Science,

5 thoughts on “The Power of the Circadian Clock and the Sun”

  1. yo maxify! i was wondering if you had any input on how getting away from phones and computers as a whole are nearly impossible to escape from in day to day life. with email and text being such crucial forms of communication, how this could also potentially be linked to suicide rates on the rise. anyways love the article and it looking forward to whatever you’ve got next

    1. It would be tough to remove electronics as a whole. However, I think it’s important we limit the use, especially in the evening time. Also, we need to do what we can to limit the addictive behavior and overall exposure.
      That’s an interesting thought. Although I haven’t looked into any research supporting that, it’s very much so possible.

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