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What Is the Circadian Rhythm?



*Nothing in this article constitutes medical advice. Seek the guidance of a physician if you have any questions.*

In 2017 Jeffrey C. Hall, Michael Rosbash, and Michael W. Young were awarded the Nobel prize in physiology or medicine. Their research centered around an often-studied yet poorly understood concept: the circadian rhythm. Hall and his team successfully discovered the molecular mechanisms that underpin these extremely important biological clocks. Today, let’s take a closer look at what exactly circadian rhythms are and why they are important.

Circadian Rhythm Basics

In the simplest terms, circadian rhythms are natural, biological processes that regulate the sleep-wake cycle roughly 24 hours. If you’ve ever flown a far distance east or west, you’re probably familiar with the concept of circadian rhythm, even if you didn’t know it!

For example, let’s say that you fly from London to New York, and you leave at 2 PM. The flight duration is eight hours, and the time change is five hours. When you land, the clock will read 5 PM, so it’s time to go out for a night on the town, right? You will quickly realize that, even though the clock says 5 PM, your body thinks that it’s still 10 PM, so you will certainly be tired.

This is just one specific example of how circadian rhythms affect our day-to-day lives, but really any process that is dependent on the rising and setting of the sun based on a 24-hour day on earth is likely to be part of the circadian rhythm.

History of the Circadian Rhythm

Humans have known about (or at least discussed the existence of) circadian rhythms for thousands of years. The concept of a “body clock” or “natural rhythm” has existed in Chinese texts since the Middle Ages. More often this concept was discussed in regard to flowering plants and nocturnal animals, though.

Fast forward to the 20th century in 1959, and the Romanian-American scientist Franz Halberg coined the term “circadian.” It derives from the Latin for “about” (circa) and “day” (dies). In Halberg’s own words, he defines this term as “it may serve to imply that certain physiologic periods are close to 24 hours, if not exactly that length. Herein, "circadian" might be applied to all "24-hour" rhythms, whether or not their periods, individually or on the average, are different from 24 hours, longer or shorter, by a few minutes or hours.”

Lastly, in 1977, the International on Nomenclature officially adopted a new, finalized definition for circadian: “relating to biologic variations or rhythms with a frequency of 1 cycle in 24 ± 4 h. Note: term describes rhythms with an about 24-h cycle length, whether they are frequency-synchronized with (acceptable) or are desynchronized or free-running from the local environmental time scale, with periods of slightly yet consistently different from 24-h”

All of this fancy definition boils down to our previous understanding: circadian rhythms are patterns and processes that are intertwined with the 24-hour cycle of days that the earth experiences.


Circadian rhythms are massively important for most life on Earth. Let’s take a look at some of the most interesting reasons why.


When genes that control the circadian rhythm are deleted, it can have drastic effects on the test subject. A 2013 study in mice showed that, when genes associated with the circadian rhythm were deleted, the mice tended to become obese and process glucose differently. This is likely due to the hypothesis that the hunger/satiation response in animals is regulated by the circadian rhythm.


The entire life cycle of most plants is based on their circadian rhythm! In the fall, leaves begin to change color and eventually drop. This is due to the changing amount of daylight that the plants receive - an important circadian rhythm regulator. Many other kinds of plant behaviors are also regulated by circadian rhythms - flowering, budding, and fruiting are some common examples.

Circadian Rhythm Disorders

As we’ve discussed, the circadian rhythm is extremely important in practically every organism on Earth. As one could imagine, having an improperly regulated circadian rhythm can lead to a whole host of problems. Let’s take a look at some of the morn common disorders.

Advanced Sleep-Wake Phase Disorder (ASWPD)

This disorder is caused by a “shift” in a patient’s circadian rhythm. The “advanced” part of the name implies that the individual’s specific rhythm is shifted forward. This means that the patient may have a hard time staying awake at night, and they make wake up too early in the morning.

Delayed Sleep-Wake Phase Disorder (DSWPD)

This disorder is the more common cousin to advanced sleep-wake disorder. As the name implies, individuals will experience a “delay” in their circadian rhythm. This will cause them to have a hard time falling asleep at night and a difficult time getting up in the morning.

Irregular Sleep-Wake Phase Disorder

Irregular sleep-wake phase disorder is the middle ground between ASWPD and DSWPD. Individuals with this condition have a circadian rhythm that regulates their sleep-wake cycle irregularly and often in chunks. Such individuals may experience parts of the day when they are extremely tired - regardless of how much sleep they got or how much caffeine they consumed. Likewise, those people may experience periods of extreme wakefulness during the nighttime as well.

Shift Work Disorder

As the name implies, shift work disorder affects workers who primarily work overnight and rotating schedules. Especially for those who work overnight, their natural circadian rhythms are in conflict with their work schedules. This can cause individuals to both feel sleepy at work and awake during the day when they can actually sleep.



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