Circadian Rhythm - The Circadian System Explained

Quick Summary

Almost everything in our body happens rhythmically, ensuring good health. There are seasonal rhythms, daily rhythms, and much shorter rhythms.

Word circadian translates from Latin as “around the day.” Thus circadian system represents a daily/24 hours cycle. It is also called circadian rhythm, and terms are used interchangeably. Circadian clock controls the timing of various bodily functions. It ensures the things happen on time like sleeping, walking, eating, and much more.

There are shorter rhythms called ultradian, like blinking, heartbeat, respiration.

Rhythms that last longer than a day are called infradian. A good example is a menstrual cycle.

Circadian system basics-biological clock, master clock, and more

Circadian clock is essential for good health. If it is disturbed, it may lead to diseases, weakening of the organism. Light has lots to do with the formation of circadian clock. Humans, by nature, are made to stay awake in sunlight and sleep in its absence.

Circadian system is not limited to people. It is found in all the mammals, even in plants and microbes. Slowly, we realize the importance of the circadian rhythm. Chronobiology is an emerging field that studies these rhythms of life.

Is the Biological Clock The Same?

The biological clock is related to circadian clock but is not the same. If a circadian system is visible changes in health and behavior, the biological clock is the mechanism at the cellular level. If the light is an external factor affecting circadian rhythm, the biological clock is an internal enabling factor. Several genes are known to control the formation of the biological clock.

So, biological clock enables the working of circadian rhythm at the cellular level. The Master clock regulates working of both these clocks.

Master clock is a name for a brain structure called the suprachiasmatic nucleus (SCN). SCN is made up of about 20,000 neurons and is responsible for keeping all the clocks in sync. It is the central control and coordination point.

How is my circadian rhythm produced or controlled?

Both internal body factors and external factor play an essential role in the formation of circadian system. Internal factors are genes, hormones, other cellular signals, control by SCN.

Environmental factors that can influence circadian clock are light, sounds, food, stress, illness, medications, and substance abuse (e.g., alcohol). See Figure 1.

If a circadian clock is disturbed, sleep is disturbed. However, that is not all. Now we know that with circadian rhythm lots may change. This rhythm also controls our body’s metabolism. It means that at certain times the body produces more energy. While at other times it focuses on the production of other substances.

It circadian rhythm is disturbed; the whole body suffers as various processes do not work in sync with each other. It could mean producing more energy when it is not required, or producing more healing chemicals when they are not needed.

Our whole body, each cell, every organ follows a circadian rhythm. It is the way of ensuring that the heart, liver, kidneys, and other organs work at optimal levels when required.

Because of higher metabolism rate during the day, our body temperature is a bit higher when compared to late night or early morning. It explains why doing physical activity during the night is uncomfortable. If we do physical work at night, our body may alter circadian rhythm. It explains the health risks involved when working in shifts (especially night shifts).

circadian system diagram

Figure 1 factors that influence circadian rhythm ( source: Brainard J, Gobel M, Bartels K, Scott B, Koeppen M, Eckle T. Circadian rhythms in anesthesia and critical care medicine: potential importance of circadian disruptions. Semin Cardiothorac Vasc Anesth. 20)

Circadian system- individual differences and relation to age

How often were you asked, are you a lark or owl?  We know that some people love to get up early, while others like to go to bed late. It is not just a matter of sleeping habits. Research seems to indicate that it is something controlled by circadian rhythm, which depends on genetics.

Circadian rhythm changes with age. Growing bodies need to rest more. Thus children sleep more than young adults, while old people sleep even less. Age-related change in circadian clock is not limited to sleep duration. As people age, they tend to get up early. Whereas, young people prefer to stay late in bed.

Circadian cycle in healthy individuals also changes with the season. During summer days are longer, means more sunlight, the opposite is right for winters. It means in summer people may prefer to sleep less. While on dark winter day people may sleep more.

These seasonal and age-related changes affect the secretion of hormones like melatonin, serotonin, and even testosterone. It means that circadian rhythm affects numerous aspects of behavior.

So now you know that why we want to have sex at certain times and prefer to have rest at other times.

Changes in circadian rhythm and health

Animals, plants, microbes, all living beings follow a biological clock. It is something long known to biologists. They know that chlorophyll in plants produces energy during the day. However, the role of daily alterations in human health long remained less appreciated.

But now we know for more than sure that such rhythm exists. Master clock controls the biological clock, affecting circadian rhythm or our behavior. There are changes in the production of hormones, heat production, energy production, during twenty-four hours. If we do not act in sync with this biological rhythm, health may suffer. See Figure 2.

Circadian Diagram 2

Figure 2 Circadian rhythm and metabolism (Source: 1. Huang W, Ramsey KM, Marcheva B, Bass J. Circadian rhythms, sleep, and metabolism. J Clin Invest. 2011;121(6):2133-2141. doi:10.1172/JCI46043)

Our body releases fewer hormones like insulin in the evening or night. Therefore, if a person often eats at night, he or she is more probable to develop insulin resistance. Similarly, change in circadian cycle may affect immunity, heart health, lung health. It is also related to reduced insulin sensitivity, diabetes mellitus, obesity, cardiovascular diseases, and much more.

Research also shows that specific treatments would work better when provided at a particular time of the day. See Figure 3.

Circadian Diagram 3

Figure 3 Circadian rhythm, disease, and treatment ( source: Brainard J, Gobel M, Bartels K, Scott B, Koeppen M, Eckle T. Circadian rhythms in anesthesia and critical care medicine: potential importance of circadian disruptions. Semin Cardiothorac Vasc Anesth. 20)

Taking care of circadian system may keep you healthy

Changes in your internal clock may disrupt the working of any organ. This disruption may cause disease of just any organ. It may cause metabolic disorders, heart diseases, mood changes, and even increase the risk of cancer.

You can do many things to correct your circadian rhythm:

  • Get adequate sun exposure during the day, while avoiding bright lights late in the evening. Blue light emitted by screens of modern gadgets is also known to alter sleep pattern.
  • Have enough sleep and follow strict sleep hygiene.
  • Avoid drinking too much alcohol and other mind-altering substances.
  • Go to bed at fix time each day.
  • Eat meals at a fixed time.

Additional Resources

  1. Pevet P, Challet E. Melatonin: Both master clock output and internal time-giver in the circadian clocks network. Journal of Physiology-Paris. 2011;105(4):170-182. doi:10.1016/j.jphysparis.2011.07.001
  2. Huang W, Ramsey KM, Marcheva B, Bass J. Circadian rhythms, sleep, and metabolism. J Clin Invest. 2011;121(6):2133-2141. doi:10.1172/JCI46043
  3. Brainard J, Gobel M, Bartels K, Scott B, Koeppen M, Eckle T. Circadian rhythms in anesthesia and critical care medicine: potential importance of circadian disruptions. Semin Cardiothorac Vasc Anesth. 2015;19(1):49-60. doi:10.1177/1089253214553066
  4. Circadian Rhythms. https://www.nigms.nih.gov/Education/Pages/Factsheet_CircadianRhythms.aspx. Accessed November 1, 2018.
  5. Potter GDM, Skene DJ, Arendt J, Cade JE, Grant PJ, Hardie LJ. Circadian Rhythm and Sleep Disruption: Causes, Metabolic Consequences, and Countermeasures. Endocr Rev. 2016;37(6):584-608. doi:10.1210/er.2016-1083
  6. Circadian rhythm. ScienceDaily. https://www.sciencedaily.com/terms/circadian_rhythm.htm. Accessed November 1, 2018.