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Circadian Rhythms: What they are and Why they matter

Ever experienced jetlag? Do you get sleepy in the afternoon? Does your work or school start too early for you? Have you noticed that some medicines are to be taken at specific times of day? All of these are experiences are a result of your circadian rhythms.

You’ve probably heard the terms circadian rhythms, biological rhythms, chronobiology, or body clocks, but they are not usually explained. This article will cover what circadian rhythms are and why they are so important.

What are circadian rhythms?
The field of circadian rhythms studies how organisms control the timing of various internal biological events and how they take in cues for the environment (such as light and food). These environmental cues are used to coordinate an organism’s internal biological rhythms with its surroundings. It is a large field with research at every level including behavior, physiology, cellular and molecular biology, and genetics; all examined in a wide variety of organisms including humans.

Circadian is Latin for about a day (circa = about, dian = day). Circadian rhythms refer to approximately 24-hour rhythms in many aspects of biology. Almost all living organisms have biological clocks including bacteria, fungi, plants, and animals (humans, mice, bats, flies, etc). In humans, biological clocks coordinate the timing of behavior (sleep-wake cycles, eating, activity, mood, cognition, etc.), physiology (metabolism, hormone release, blood pressure, body temperature, etc.), and individual cell function (DNA repair, cell cycle, etc) to allow for the body to work properly [1]. Put simply, biological clocks make sure that everything is happening in the right place at the right time of day.

In mammals, almost every cell in the body has its own molecular clock. The circadian rhythms of an organism is created by a network of these single-cell clocks. A region of the brain, known as the suprachiasmatic nucleus (complicated sounding name for where it is located in the brain) coordinates all of the clocks in your body and is considered the master clock.

The master clock is able to produce and maintain circadian rhythms in the absence of any environmental cues. So if you were put in a cave with food and resources (yes, scientists really did this to themselves [2]), and had no clues about the time of day from your environment, your body would still follow an approximately 24-h rhythm. As your internal clocks run on their own time (a little more or less than 24 hours), over time you will become completely desynchronized from the environment. But, because we usually do have access to environmental cues such as light, food and beverages, and physical/social activity, our circadian system incorporates those cues to coordinate the biological rhythms with the environment.

The master clock also receives feedback signals from clocks throughout the body. In this way, it is in an integration center for both internal and environmental time.  

Why are circadian rhythms important?
Circadian rhythms are a core part of physiology. Disrupting the circadian system either behaviorally (jet lag or shift work, actual or simulated in a lab), or genetically (breaking or altering the molecular clock), compromises the health and lifespan of a variety of organisms [3].

In humans, when circadian rhythms are disrupted due to shift work or chronic jet lag, incompatible behavioral and physiological events can coincide. Repeated disruption of biological rhythms by shift work and sleep disruption can lead to an increased risk for a wide variety of diseases including heart disease, diabetes, cancer, obesity, depression, bipolar disorder, and sleep disorders [4].

The circadian system is a core aspect of physiology and crucial for our health. Many studies have found that disrupting the circadian system increases the risk of disease, whereas supporting circadian rhythms with consistent environmental input can support health.

Terminology
Circadian Rhythms: (Circa = Approximately; Dian = Day). Biological rhythms are often also referred to as circadian rhythms because many biological rhythms have about a 24-hour period (~one day). However, there are also biological rhythms that are shorter or longer than 24 hours such as ultradian rhythms (shorter than a day), circalunar rhythms (~a month), and circannual rhythms (about 1 year).

Biological Rhythms: rhythms in behavior or physiology such as sleep-wake cycles, metabolism, and body temperature.

References

  1. Bass, J. and Lazar, M.A., 2016. Circadian time signatures of fitness and disease. Science, 354(6315), pp.994-999.
  2. Azvolinsky, A. (2016, March 1). Cave Dwellers, 1938. Retrieved from https://www.the-scientist.com/foundations/cave-dwellers-1938-33966
  3. Potter G.D.M. et al., 2016. Circadian Rhythm and Sleep Disruption: Causes, Metabolic Consequences, and Countermeasures. Endocrine Reviews 37(6) pp.584–608.
  4. Arendt, J., 2010. Shift work: coping with the biological clock. Occupational Medicine, 60,(1), pp.10–20.