Article at a Glance: 

• The human body has a master clock that orchestrates many circadian rhythms which follow an approximate 24-hour period to help keep the body healthy.
• Light and darkness are important cues that alert the body when to carry about biological functions, such as releasing hormones that tell your body when to be alert and when to go to sleep. 
• When the body shifts its sleep/wake cycle, circadian disruption can occur. 
• Shift work, traveling across time zones, and overexposure to artificial junk light can cause circadian disruption, and, in turn, lead to many short-term and long-term health consequences. 

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Like most people, your body probably naturally wakes up, gets hungry, and feels tired at the same time each day. If you’ve ever wondered why you have circadian rhythms to thank. These daily cycles are essential for regulating important biological functions that help keep you healthy.  Research shows that [chronic] misalignments between your natural circadian rhythms and your lifestyle can negatively impact your well-being and put you at greater risk of illness and disease.

So, what are circadian rhythms?

The truth is that there is no clear definition. Some refer to this as a single “biological clock” or the “sleep/wake cycle”, but there’s actually a lot more going on inside of our bodies besides just sleep and wakefulness.  From scientists’ perspective, circadian rhythms are physical, mental, and behavioral changes that follow an approximately 24-hour cycle. ⁽¹⁾

Dr. Satchin Panda, a circadian biologist at the Salk Institute for Biological Studies, says: 

The bottom line is that almost every hormone, every brain chemical, every digestive enzyme, and so on is pre-programmed to peak at a certain time of the day and then tap out at another time of the day. It’s an in-built schedule for different programs to do different things at the optimal time, and these timing mechanisms are the circadian rhythm[s]. 

These rhythms are present in most living things — including animals, plants, and humans – and they are essential for regulating critical functions such as behavior, hormone levels, sleep, body temperature, and metabolism. ⁽³⁾

How do circadian rhythms work?

Thanks to three scientists (that won the 2017 Nobel Prize in Medicine) we now have a better understanding of the molecular mechanisms that influence circadian rhythms. In studying fruit flies, the scientists discovered a gene-encoded protein that oscillates between high levels at night and low levels during the daytime, in synchrony with a 24-hour cycle. Their findings support the notion that circadian rhythms primarily respond to light and darkness, and that they help the body’s master clock adapt human physiology to the different phases of the day. 

This makes a lot of sense if you think about the natural progression of the sun’s position in the sky throughout a 24-hour period. Each morning, the sun’s natural light – particularly blue light — enters your eyes and signals to your brain that it’s time to wake up.  By mid-day, your body is at peak alertness while the sun is at its highest point in the sky. And when the sun descends below the horizon each evening, your body naturally begins producing more melatonin so that you can wind down and fall asleep while it’s dark outside.   

This natural cycle of light to dark and back directly influences the genes that speed up, slow down, or reset your circadian rhythms. Your body not only anticipates this oscillation, but it also adapts to it so that you can feel and function your best.

What is the “master clock” and how does it influence circadian rhythms?

For circadian rhythms to occur, the body must have a mechanism to interpret light and darkness. That’s where intrinsically photosensitive melanopsin ganglion cells (ipRGCs) and the suprachiasmatic nucleus (SCN), or master biological clock, come in.  

ipRGCs are a subset of cells that participate in visual responses and notably have distinct sensitivities and responses to light. ⁽⁴⁾ These cells convey irradiance input centrally via the optic nerve and stimulate the SCN.⁽⁵⁾ 

The SCN is a bilateral structure located in the brain’s hypothalamus.  It contains roughly 20,000 neurons that interpret light input from ipRGCs to entrain circadian rhythms in rest and activity, core body temperature, neuroendocrine function, autonomic function, memory and psychomotor performance, and a host of other behavioral and physiological processes. ⁽⁶⁾ 

Just like the ebb and flow from daylight to darkness, your body needs this master clock to orchestrate each of the smaller clocks throughout the body’s tissues and cells.  Together, your circadian rhythms help regulate your mood, cognitive function, heart health, stress levels, and immune system.

The Dangers Associated with Disruptions to Your Circadian Rhythms

Disharmony between your internal circadian clocks and environmental cues is referred to as circadian disruption. ⁽⁷⁾ Circadian disruption affects many biological processes within the body and can have numerous short-term and long-term health consequences. 

Short-term health concerns:

• Fatigue 
• Weight gain 
• Impulsivity 
• Slower thinking and reaction times 
• Mood disorders⁽⁸⁾ 
• Disrupted glucose metabolism and obesity⁽⁹⁾

Long-term health concerns:

• Cardiovascular disease⁽¹⁰⁾ 
• Diabetes⁽¹¹⁾ 
• Cancer⁽¹²⁾ 
• Alzheimer’s disease⁽¹³⁾ 
• Parkinson’s disease⁽¹⁴⁾

Various external factors can lead to a mismatch between our external environment and this internal biological clock. For example, traveling across several time zones can result in “jet lag” symptoms that leave us feeling fatigued and weak. Shift work – working outside of the typical 9-5 window – has been identified as a “probable carcinogen” and culprit for chronic circadian disruption. ⁽¹⁵⁾  Overexposure to artificial junk light, stemming from the increased use of LEDs, fluorescents, and digital devices with LED screens, can also throw the body’s master clock and circadian rhythms off-kilter, making it difficult to fall asleep at night and reach peak performance during the daytime. 

The bottom line: sticking to a regular schedule every day is important for regulating your mood, cognitive function, stress levels, sleep and immune health, and more. One of the best things that you can do to keep your biological rhythms in sync is to manage your light exposure so that your body knows what time of day (or night) it is, and how to adapt.

References

1. https://www.nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms.aspx
2. https://www.theverge.com/2018/6/12/17453398/sleep-circadian-code-satchin-panda-clock-health-science
3. https://www.nobelprize.org/prizes/medicine/2017/press-release/
4. https://science.sciencemag.org/content/366/6470/1251
5. https://pubmed.ncbi.nlm.nih.gov/15088713/ 
6. https://www.sciencedirect.com/topics/neuroscience/suprachiasmatic-nucleus 
7. https://www.researchgate.net/publication/308964320_Carcinogenic_effects_of_circadian_disruption_An_epigenetic_viewpoint 
8. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3725187/ 
9. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5142605/ 
10. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4632990/ 
11. https://pubmed.ncbi.nlm.nih.gov/21921296/ 
12. https://www.researchgate.net/publication/308964320_Carcinogenic_effects_of_circadian_disruption_An_epigenetic_viewpoint 
13. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4351409/ 
14. https://www.sciencedirect.com/science/article/pii/S2451994416300177 
15. https://www.thelancet.com/journals/lanonc/article/PIIS1470-2045(19)30455-3/fulltext