Circadian Rhythm: What it is and Why it Matters

Mary Sweeney - Updated on March 22nd, 2023

Sleep Quality and Quantity

You will have noticed a recurring theme on our site—the circadian rhythm. This important function of our body is responsible for sleep quantity and quality, regulating our sleep cycles, and telling us when it’s time to wake up and go to sleep. So, let’s take an in-depth look at this incredible system!

What is the Circadian Rhythm?

To put it simply, the circadian rhythm is a series of physical, mental, and behavioral changes that the body goes through in a daily cycle. This rhythm regulates many things, from our sleep cycles to our body temperatures. Fun fact: humans aren’t the only ones with a circadian rhythm! Plants and even tiny microbes have a certain circadian rhythm as well.

How Does it Work?

The circadian rhythm is controlled by a part of your brain called the hypothalamus. The hypothalamus receives signals based on the amount of light you’re exposed to. For example, when it’s dark outside, your eyes send a signal to the hypothalamus that it’s time to go to bed. Your hypothalamus then sends signals to your body to produce melatonin, the hormone that is responsible for making you feel tired. Conversely, when it’s light outside, your body knows to produce less melatonin. This makes you feel more awake. 

Biological Clock vs. Circadian Rhythm

It is commonly thought that the circadian rhythm in humans is the same as a biological clock—this is not true. The human biological clock is made up of small proteins that interact with all different types of cells in the body, which makes up this unique “timing device” in humans. It is also what regulates the circadian rhythm. Lots of different factors can affect the body’s biological clock, the most common of which is daylight.

The Sleep Cycle

If you’ve taken a look at any of our other articles, you’ve probably noticed that we really like talking about the sleep cycle. The sleep cycle is important to understand here, because it’s a cycle that our body goes through as a result of our functioning circadian rhythm. Here’s the timeline of events—with daylight receding, our biological clocks are stimulated to activate the circadian rhythm. The circadian rhythm stimulates the body to produce melatonin, which makes us tired. We feel tired and realize it’s time for bed. We close our eyes and fall asleep, thus entering the sleep cycle. Here’s a breakdown of what happens after we fall asleep:

A normal sleep cycle lasts about 90 minutes and can be broken down into two different types: rapid eye movement (REM) sleep and non-REM (NREM) sleep. Within those 2 different types, there are 4 stages of sleep.

Stage 1: This first stage of NREM sleep is light sleep, lasting only a few minutes or so. Here, your body is preparing to fall into a deeper sleep, your eye movements slow down, and your brain begins producing alpha and theta waves. During this stage, you can be awoken easily, as you’re only lightly sleeping.

Stage 2: During this NREM stage, you’re still lightly sleeping but your heart rate slows down, your muscles relax significantly and your body temperature decreases slightly. Eye movements will slow down during this stage, and your brain waves slow down with occasional increases in activity.

Stage 3: This NREM stage is restorative sleep, the kind that makes you feel refreshed. If someone were to wake you up during this stage, you would likely feel disoriented for a few minutes before recognizing your surroundings. Our eye movements slow down or stop, and it’s very difficult to wake us up. This stage is an important one—it’s where your body starts repairing muscle, strengthening your immune system, and other vital processes.

Rapid eye movement (REM) sleep: During REM sleep, our eye movements increase dramatically, as the name suggests. REM sleep is the stage where you dream the most, and it’s thought that the eye movements are related to the dreaming that you’re doing. During this stage, most of your muscles are in a paralyzed state, to keep you from acting out your dreams. Don’t worry though, the important muscles like your heart and diaphragm aren’t paralyzed and are working like they’re supposed to!

While the average sleep cycle lasts 90 minutes, you will go through several cycles of varying lengths over the course of one night. During your first few sleep cycles, you’ll go through longer cycles of NREM sleep, followed by a few cycles of REM sleep. As if that doesn’t complicate things enough, research shows that the time of day can affect what type of sleep you get. For example, it is thought that most NREM sleep happens between the hours of 11 pm and 3 am, and REM sleep more often happens between 3 am and 7 am.

The above sleep cycle assumes that you are someone who goes to bed at night and wakes in the morning. As you might imagine, the sleep cycle would differ with night shift workers or those who suffer from insomnia or other sleep disorders.

Circadian Rhythm Disorders

There are a few types of circadian rhythm disorders that can interrupt the important sleep-wake cycle that our bodies need to function daily. According to the Merck Professional Manual on sleep disorders, “Circadian rhythm sleep disorders are caused by desynchronization between internal sleep-wake rhythms and the light-darkness cycle. Patients typically have insomnia, excessive daytime sleepiness, or both, which typically resolve as the body clock realigns itself. Diagnosis is clinical. Treatment depends on the cause.” Here are the three main types of circadian rhythm disorders:

Jet Lag Disorder

We all have probably experienced this at one point or another in our lives. If you’ve traveled across two or more time zones, you probably have had a hard time adjusting your body to its new sleep schedule. According to Merck Manuals, jet lag is worsened with eastbound travel because it advances the sleep cycle. Traveling westbound delays the sleep cycle and therefore causes less severe symptoms.

To avoid jet lag disorder, here are a few things you can do:

  • Gradually shift your sleep-wake cycle the week before you travel. Shift your bedtime by a few minutes each night until you are on the new schedule.
  • Maximize your exposure to daylight when you arrive and exposure to darkness before bed. This will stimulate the production of the proper amount of melatonin your body needs to feel tired.
  • Use short-term, fast-acting sleep aid medications to help you adjust, but only after consulting with your doctor to make sure it’s safe. These medications include Ambien (prescription only) or Benadryl (over the counter).

Shift Work Disorder

People who work evening and night shifts can have a particularly hard time achieving a quality night’s sleep. Add in a rotating shift schedule and it can get even harder. According to Merck Manual, the severity of shift work disorder symptoms can vary based on:

  • How often you change shift schedules.
  • How drastic the shift schedule change is.
  • The number of consecutive nights worked.
  • The length of the shift you work.

While there may not be much you can do about what shifts you work, there are some things you can do to mitigate the side effects of rotating shifts. These things include:

  • If possible, try to work fixed shifts—this means trying to work a block of evenings followed by a block of nights, etc. If you have to work a rotating shift schedule, try to rotate clockwise. This means working days to evenings, evenings to nights, then nights to days. 
  • Maximize your exposure to bright light when you should be awake and maximize your exposure to darkness when you should be sleeping. This may mean exposing yourself to artificial light if you work at night and getting blackout curtains and an eye mask to help you sleep during the day. 

Circadian Rhythm Sleep Disorder

There are a few subtypes to this category of sleep disorder. They are:

Delayed Sleep-Wake Phase

In this disorder, the sleep cycle is delayed by two or more hours. This results in a later bedtime and a later wake time in the morning.

Advanced Sleep-Wake Phase

This disorder causes people to fall asleep much earlier than their normal bedtime, then wake much earlier than their usual wake time in the morning.

Irregular Sleep-Wake Rhythm

With this disorder, a person has no real sleep cycle or pattern. Often, they may take a series of naps over a period of 24 hours.

Non-24-Hour Sleep-Wake Rhythm

Here, a person may have frequent changes to their sleep patterns that may not be noticeable until the patterns do not align with the patterns of others. This may affect people who are nearly or totally blind.

To treat any of the above circadian rhythm sleep disorders, melatonin supplements or melatonin antagonist medications may be helpful. It’s best to consult a doctor if you think you may be suffering from any of the above disorders. They may be able to rule out other causes of your sleep issues and will be able to direct you to the best course of treatment.

References and Continued Reading