The Circadian Rhythm: Timing is Everything

Posted on by Jesse Santiano, M.D.

Circadian means “about a day” in Latin and has a 24-hour cycle. It is essential to know about circadian rhythms because there are times in a day when specific actions are optimal. An example is working during the day and resting during the night.

If the activity and the circadian rhythm is out of timing, then diseases like hypertension, diabetes, obesity, and depression can develop.

How Does the Body Set Time?

The atomic clock in Colorado sets the time standard for the whole U.S., including the GPS system. The human body also has a master clock.

Right behind the eyes lies the body’s master time-keeper, the suprachiasmatic nucleus, or SCN. The light received by the eyes is sent to the SCN, the master clock. The master clock sets the time standard, and the light signal is then to the pineal gland.

If there is light, no melatonin is produced, and the body wakes up. As darkness comes, melatonin is produced and causes sleepiness.

1280px-Circadian_rhythm.svg

All internal organs have clocks too. The SCN dictate the time to all the peripheral clocks. Each peripheral clocks regulate organ functions like hormone secretions, blood pressure,  digestion, and many others.  That is why certain activities are performed better at certain times of the day, as shown in the image below.

1920px-Biological_clock_human.svg

Adjusting the Circadian Rhythm

If we travel from one time zone to another, we have to correct our watches with the local time. The body does the same by adjusting its circadian rhythm based on the presence or absence of light. That is an example of entrainment.

Entrainment is the property of the circadian rhythm to be synchronized to an external stimulus. Light is the most common and most significant modifier of the circadian rhythm and is a zeitgeber.

Zeitgeber (click here for pronunciation) means “time-giver” or “synchronizer” in German. It is a term used in circadian rhythms for an external cue that entrains or influence the circadian rhythm. The circadian rhythm can be advanced or delayed as it adjusts to a new time zone.

Other zeitgebers are social interactions, work, medications, alcohol drinking, exercise, and temperature. A zeitgeber can promote health or cause diseases.

This short article serves as an introduction to circadian rhythms and zeitgebers. Future articles will discuss how zeitgebers can be used to prevent diseases.

Take Away Message

  1. Activities that are out of synch with the circadian cycle can lead to disease conditions. An example is working at night.  Shift work, like smoking, is a significant risk factor for developing a heart attack.
  2. Adjusting what we do to the body’s circadian rhythm is healthy by making the body functions work efficiently, thus minimizing stress.
  3. Zeitgebers can be used to synchronize the circadian rhythm to each personal schedule to prevent diseases.

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References

  1. Fukuhara, Chiaki & Tosini, Gianluca. (2003). Peripheral circadian oscillators and their rhythmic regulation. Frontiers in bioscience: a journal and virtual library. 8. d642-51. 10.2741/1042.
  2. Mohawk, Jennifer A et al. “Central and peripheral circadian clocks in mammals.” Annual review of neuroscience 35 (2012): 445-62
  3. Bass J1, Takahashi JS. Circadian integration of metabolism and energetics. Science. 2010 Dec 3;330(6009):1349-54. doi: 10.1126/science.1195027.

Image Credits:

  • Circadian Rhythm By NoNameGYassineMrabetTalk✉ fixed by Addicted04 – The work was done with Inkscape by YassineMrabet. Information was provided from “The Body Clock Guide to Better Health” by Michael Smolensky and Lynne Lamberg; Henry Holt and Company, Publishers (2000). The landscape was sampled from the Open Clip Art Library (Ryan, Public domain). Vitruvian Man and the clock were tested from Image:P human body.SVG (GNU license) and Image: Nuvola apps clock.png, respectively., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3017148
  • Eyes Pineal Gland and Melatonin By NoNameGYassineMrabetTalk fixed by Addicted04 – The work was done with Inkscape by YassineMrabet.
  • Phase Response Curve. Bass, Takahashi. Circadian integration of metabolism and energetics. Author Manuscript

 

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