August 08, 2023 5 min read

If there is one thing that affects your physiology and behavior the most, it’s Earth’s rotation around its axis. 


The resulting 24-hour changes in light and darkness have been a major feature of life on Earth for hundreds of millions of years. As a consequence, virtually all life on this planet has developed an internal clock to rhythmically follow these changes, and humans are no exception. 


Called circadian rhythms, these daily shifts in physiology and behavior need to work well for us to be healthy and functional. Here is more on circadian rhythms and why they are so important to our ability to function.

What Are Circadian Rhythms?

Circadian rhythms are physiological and behavioral changes that follow a 24-hour cycle [1]. Examples of circadian rhythms include: 


  • The sleep-wake cycle
  • Daily changes in brain wave activity (and alertness)
  • Daily changes in core body temperature
  • Shifts in hormone production and cell regeneration

In other words, your circadian rhythms dictate when you sleep, eat, and go about your life. 


These rhythms are regulated by the body’s internal clock in response to environmental cues. The body’s internal clock, or the circadian pacemaker, is located in the brain. It is known as the suprachiasmatic nucleus (SCN), a small region of the brain containing 20,000 nerve cells found in the hypothalamus. Clock genes within the SCN send signals throughout the body in response to changes in light but also temperature and activity levels. 


The purpose of circadian rhythms is to regulate physiology and behavior and to coordinate different processes in a way that keeps you healthy, safe, and functional. These rhythmical patterns slowly establish themselves after birth. Newborns need time to adjust their biology to environmental cues, which sleep and eating patterns tend to be erratic in the first 2 to 9 months of life. 

Circadian Rhythm vs. Biological Clocks

While the terms “circadian rhythm” and “biological clock” are often used interchangeably, they’re not the same thing. Circadian rhythms are patterns that are the result of the activity of biological clocks, like the SCN. Explained differently, biological clocks are innate timing “devices” that regulate the patterns of circadian rhythms. 


The master clock is the SCN, but it is not the only clock. Nearly every organ in the human body — the liver, pancreas, muscles, and even fatty tissue — has cellular clocks that use molecules to communicate. 


The exact mechanisms that regulate our biological clocks were a mystery until a major scientific breakthrough a couple of years ago. In 2017, researchers Jeffrey C. Hall, Michael Rosbash, and Michael W. Young won the Nobel Prize for their discoveries of the molecular mechanisms behind circadian rhythms and biological clocks. Using fruit flies, they found a gene that regulates these processes. More specifically, this gene encodes a protein that accumulates in cells during the night and degrades during the day [3].

Circadian Rhythm and Sleep

The sleep-wake cycle is the most obvious example of a circadian rhythm. Humans have internal clocks that are fine-tuned to the daily changes in light and darkness. However, we also respond to many other environmental cues — all called zeitgebers — like drops in temperature, rainfall, social activity, food availability, etc.


These zeitgebers work as signals for our SCN to tell the glands to release hormones — notably melatonin — that make us sleepy when dusk falls and to shift physiological processes. Some other shifts that occur when you sleep are [3]:

  • Drops in core body temperature
  • Drops in heart rate and blood pressure
  • Rise in growth hormone levels
  • Memory consolidation and learning 
  • Metabolism regulation
  • Immune system restoration

These processes are tightly controlled by the sleep-wake cycle. Unsurprisingly, disruptions in this cycle lead to aberrations in these physiological processes that result in disease [4]. 

Disruptions in Circadian Rhythms

For many of us, our modern lifestyles, unfortunately, clash with our circadian rhythms, which can cause health problems. Changes in our bodies can also cause our physiological processes to go out of sync. Disruptions in our circadian rhythms can happen as a result of: 


  • Shift work or working flexible hours
  • Frequently traveling over different time zones
  • Constant light from electronic devices at night
  • Poor sleep hygiene
  • Mental and physical health problems
  • Unmanaged or chronic stress
  • Certain medications, drugs, and stimulants


When our sleep patterns are out of sync with our circadian rhythm, this is known as circadian rhythm sleep disorder. Two common types of circadian sleep disorders are advanced sleep phase and delayed sleep phase disorders [1]. These can develop as a result of lifestyle factors, neurological disorders, and blindness, to name a few. 


The immediate consequence of these disorders is being chronically tired, sleepy, and unable to function normally. If left untreated, sufferers are at a high risk of developing mental health problems, poor immunity, heart disease, type 2 diabetes, and even cancer [4]. 

How to Reset Your Circadian Rhythm

If you believe you have a circadian rhythm sleep disorder, there are therapies that can help. Current treatment options include [5]:


Chronotherapy

This is essentially resetting your circadian clock by delaying the time you go to bed and wake up by three hours every two days until your lifestyle gets in sync with your circadian rhythm. Following good sleep hygiene can help you achieve this. Sleep hygiene refers to habits that are known to promote good sleep and include sticking to a sleep schedule, avoiding stimulants before bedtime, and creating a peaceful sleep environment. 


Timed bright light exposure 

Also called bright light therapy, this option is ideal for those with delayed sleep phase syndrome. People with this problem usually fall asleep after midnight and have trouble waking up in the morning, which can interfere with normal functioning. By exposing themselves to bright light in the morning and avoiding bright light in the evening, they can gradually reset their sleep-wake cycle to match their innate biological clock. The source of light can be natural, i.e. sunlight or artificial, e.g. a spectrum lamp at 10,000 lux.


Melatonin supplements

Shift workers often have trouble falling asleep when they need to, in which case melatonin supplements can help. Melatonin is a natural hormone and neurotransmitter produced by the pineal gland located at the center of your brain. But melatonin is also widely available as a nutritional supplement and is not regulated by the FDA. When taken in the evening, melatonin advances the timing of circadian rhythms.


Takeaways

Your circadian rhythm is an important aspect of health. Our bodies are programmed to function in accordance with these natural rhythms, which are innate but respond to outside cues.

When our lifestyles and habits are out of sync with these natural rhythms, our ability to function normally declines and our health suffers. Health consequences of circadian rhythm disorders include cognitive problems, mood disorders, metabolic issues, and a poorly functioning immune system.

Paying attention to your circadian rhythm is important for health and well-being. Luckily, these problems are easy to correct with simple chronotherapy practices. 


References:


  1. Reddy S, Reddy V, Sharma S. Physiology, Circadian Rhythm. [Updated 2023 May 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from:https://www.ncbi.nlm.nih.gov/books/NBK519507/

  1. Klarsfeld A, Birman S, Rouyer F. L’horloge circadienne à l’heure Nobel - Prix Nobel de Médecine 2017 : Jeffrey C. Hall, Michael Rosbash et Michael W. Young [Nobel time for the circadian clock - Nobel Prize in Medicine 2017: Jeffrey C. Hall, Michael Rosbash and Michael W. Young].Med Sci (Paris). 2018;34(5):480-484.doi:10.1051/medsci/20183405023

  1. Irwin MR. Why sleep is important for health: a psychoneuroimmunology perspective. Annu Rev Psychol. 2015;66:143-172.doi:10.1146/annurev-psych-010213-115205

  1. Foster RG. Sleep, circadian rhythms and health.Interface Focus. 2020;10(3):20190098.doi:10.1098/rsfs.2019.0098

  1. Barion A, Zee PC. A clinical approach to circadian rhythm sleep disorders.Sleep Med. 2007;8(6):566-577.doi:10.1016/j.sleep.2006.11.017



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