What Are Circadian Rhythms?

By Wendy Innes. May 7th 2016

Circadian rhythms occur in nearly every living organism. But what are they and what do they do? Here you’ll discover what circadian rhythms are and how they work, as well as what might happen when they aren’t functioning properly.

What Are Circadian Rhythms?

Circadian rhythms are physical, mental and behavioral changes that occur on a 24-hour cycle. These cycles respond to light and darkness, primarily daylight and nightfall. While most think that circadian rhythms and the body’s internal clock are the same, they aren’t. However, they are definitely related.

The reason that people wake in the morning and sleep at night is due to the body’s circadian rhythms. These rhythms also play a role in hormone regulation, body temperature and other vital bodily functions.

In order for a rhythm to be called “circadian,” it must have certain characteristics.

  • It must occur roughly every 24 hours. Sometimes people can have problems with their circadian rhythms in which the cycle is longer or shorter. This is characterized as a circadian rhythm disturbance.
  • The cycles must be endogenous, meaning that it exists in the absence of external cues.
  • The cycles must be entrainable, meaning that it is able to adjust to local time zones.
  • The cycles must maintain and exhibit temperature compensation, meaning that the cycle period must be able to maintain itself despite temperature shifts.

How Do They Work?

Circadian rhythms depend on light and darkness to properly regulate the body. Though they are created by natural factors within the body, the environment around a person is critical to regulating the rhythms.

The body actually has several clocks as well as a “master clock” that keeps everything in sync with circadian rhythms. This master clock is a group of nerve cells in the brain called the suprachiasmatic nucleus, or SCN for short. There are about 20,000 of these nerve cells in the SCN, and it is located in the hypothalamus of the brain, which is located just above where the optic nerves cross.

Because the SCN is located in the same area of the brain as the optic nerves, the SCN responds to light cues from the environment. However, not just any light will do; it has to be the full spectrum of daylight. People who work odd shifts, like sailors on submarines, people who work nights or those working in Polar Regions, often experience problems with their circadian rhythms because of the extreme absence of sunlight.

Circadian rhythms play a role in hormone production that controls sleep and wakefulness. Melatonin, the hormone that produces sleepiness, and cortisol, the hormone that produces wakefulness, are regulated by circadian rhythms. When melatonin levels are high, cortisol levels are low and vice versa. Both these hormones are highly susceptible to retinal light exposure. Light suppresses the production of melatonin and stimulates the production of cortisol, along with the normal stresses that a person experiences throughout the day. This is the reason that when a person is experiencing an extremely stressful time in their lives, they experience problems sleeping.

Thyroid stimulating hormone (TSH) and prolactin are two more hormones that are affected by circadian rhythms. TSH is one of the hormones that regulate the function of the thyroid and prolactin is a reproductive hormone closely related to human growth hormone. Under normal circumstances, TSH rises before sleep and decline during sleep. Sleep inhibits the production and secretion of TSH. So when sleep is deprived, TSH levels remain elevated. Conversely, prolactin is stimulated by sleep and the peak of prolactin production and secretion can be measured during sleep. So it’s easy to see that when circadian rhythms become out of balance, vital hormone production is affected.

Disturbances in Circadian Rhythms

There are a number of things that can cause disturbances in circadian rhythms. The most common form of circadian rhythm disturbance is jet lag. Jet lag occurs when people cross multiple time zones and their circadian rhythms have difficulty adjusting. A person’s body may still think its 10pm, for instance, and be preparing for sleep, when in fact it’s really 7pm and it’s time for dinner. Thankfully, jet lag doesn’t last very long, as the body usually adjusts fairly quickly.

Shift work sleep disorder is another type of disturbance in the circadian rhythms that is experienced by those who work rotating shifts or night shifts. Working these odd hours requires people to be awake when the body’s natural inclination is to sleep.

Delayed sleep phase disorder and advanced sleep phase disorder are conditions that are similar, but completely opposite. People with delayed sleep phase disorder are often referred to as “night owls”. Likewise, those with advanced sleep phase disorder are “morning larks”. People with these conditions experience difficulty sleeping and waking at normal times. They either go to bed very early or very late and likewise wake very early or very late.

Resetting Circadian Rhythms

One of the most effective ways to reset circadian rhythms is with light. Light therapy uses a special light that mimics natural daylight and helps rebalance the hormones in the body that become imbalanced with the circadian rhythms in the body are not functioning properly.

A person’s behavior can greatly affect circadian rhythms. In order to maintain healthy rhythms people should maintain the same sleep times and wake times and maintaining a healthy lifestyle by eating right, exercising, and not smoking and drinking.

A disturbance in one’s circadian rhythms can be very frustrating but it isn’t the end of the world. By maintaining a healthy lifestyle and using bright light therapy, people can rebalance their bodies and return to a more normal life.


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