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What Is The Definition Of Circadian Rhythm (Body Clock)?

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Circadian is a strange word, isn’t it? Have you ever wondered where it came from?

It’s a Latin term combining “circa,” which means around, and “diem,” meaning day. That’s why we typically hear of circadian rhythms being daily in nature.

They’re also referred to as endogenous, which means built-in. So, our body clock then is a built-in mechanism that signals us when it’s time to do an activity, like sleep. This clock can be influenced by external cues, referred to as zeitgebers. The word zeitgeber is of German origin, and literally means “time giver.”

How To Maintain a Healthy Circadian Rhythm

Circadian rhythm1 refers to the biological tendency to operate in 24-hour cycles of sleeping and waking. This term is also referred to as a biological or internal clock, even though they’re not quite the same thing (we’ll discuss that in a moment).

Our body’s circadian rhythm cues us when it’s time to go to sleep and time to wake up. Depending on our age and lifestyle habits, our natural rhythm may change, but most people are usually awake for about 16 hours2 out of the day and then sleep for about 8 hours through the night (1). Throughout this time, we see peaks and valleys in our energy as well. According to Johns Hopkins Medicine3, most adults typically feel sleepiest between 2:00 and 4:00 a.m. and again between 1:00 and 3:00 p.m., which is why a nap or siesta is typical in other countries in the afternoon.

Light and darkness affect the circadian rhythm, but we still cycle through these 24-hour periods even without visual cues.

Interestingly, these 24-hour cycles, or circadian rhythms, are not unique to humans4, or even mammals. They’re also present in other animals, plants, fungi, and bacteria.

Biological Clock(s)

A biological clock is, in scientific terms, a collection of protein molecules that interact with cells throughout the body. In less scientific terms, you can think of this as a sort of clock that is responsible for regulating your circadian rhythm. Within your body, nearly every tissue and organ has its own biological clock4. This means throughout the day, often at different times of the day, tissues and organs are on a “schedule5 to execute their tasks. This is why in traditional Chinese Medicine, they use something called “The Organ Body Clock6 in order to help determine which organ is responsible for certain diseases.

All of the tissues and organs, though, and their individual biological clocks, are on the 24-hour cycle of one day.

The Master Clock

The master clock is what controls all of your body’s biological clocks, and it’s made up of 20,000 nerve cells that form a structure called the suprachiasmatic nucleus (SCN)7. The master clock syncs up all internal clocks.

This master clock is located in the hypothalamus, a part of the brain that controls many biological functions, like body temperature, thirst, and hunger. The system works by receiving information from our eyes like when it is light outside and when it is dark. Additionally, our body’s master clock is completely reliant and interwoven with the earth’s rotation.

Just as a refresher: Every 24 hours the earth rotates on its axis. When our part of the planet is facing the sun, that’s when we have daylight. When our part of the planet is facing away from the sun, it’s nighttime.

Our eyeballs, which are essentially an extended part of our brain8, detect when it is light outside and when it is dark. This communication is sent directly to the master clock, which then lets all of the biological clocks in our body know what they should be up to, including whether it is time to be sleepy (it’s dark) or awake (it’s light)4.

As you’ll see, there are plenty of reasons why you might not feel sleepy when it’s dark or awake when it’s light.

Circadian Rhythm Disorders

As mentioned, your circadian rhythm is set by certain biological clocks. What if those clocks are running too slow or fast, though? What if those clocks take a trip from New York to Paris to Thailand in two weeks, and they don’t know what’s what?

This is where Circadian Rhythm Disorders9 come in. Also called “Sleep-Wake Cycle Disorders,” this is when your body’s biological clocks are out of sync with your environment.

Circadian rhythm disorders can be long-term, caused by things like aging, genes, or medical conditions. They can also be short-term, caused by things like jet lag or shift work9.

1. Delayed Sleep-Wake Phase

A delayed sleep-wake phase disorder (DSWPD) is associated with people who refer to themselves as “night owls.” This is one of the most common circadian rhythm disorders and involves a delay in falling asleep.

Specifically, a person with this condition will have trouble falling asleep for two or more hours after a “normal” bedtime. (Usually, these people will fall asleep well after midnight.) This is because their body’s internal clock is sending alert signals, rather than “sleepy” signals, until late in the night.

These people will also usually have trouble waking up at an early hour in the morning because their bodies will not yet be producing those alert signals.

Interestingly, unless you have other sleep disorders like sleep apnea or insomnia, people with DSWPD tend to sleep well10, in terms of duration and quality of sleep. The problem is the delay in getting to sleep, which makes it difficult to wake up for things like school or work.

2. Advanced Sleep-Wake Phase

They say the early bird gets the worm, and those with advanced sleep-wake phase disorder can certainly attest to this. The only problem is that people with an advanced sleep-wake phase disorder get tired early in the evening, and they’re often awake and staring at the ceiling before morning9.

There are some positives to being on this type of schedule. You can accomplish more by breakfast than most people do all day, but if you have a social engagement planned for the evening, you’re likely to feel tired, and your yawning could be mistaken for boredom.

If you find yourself on this type of schedule, avoid the temptation to take stimulants at night or sleeping pills at 3:00 a.m. Instead, think of ways you can use this to your advantage, perhaps even exploring a career that caters to this type of schedule, like medicine or fitness.

3. Irregular Sleep-Wake Rhythm

An irregular sleep-wake rhythm is much harder to adjust to than either a delayed or advanced one. This is because there’s no real pattern to it. Likely, the person has a weak internal clock, and so they get mixed messages from their body about when it’s time to feel drowsy or awake.

This might look like several short periods of sleep throughout the night or the need to take multiple naps throughout the day. You probably won’t feel rested after you sleep9.

As you can imagine, this makes work, school, relationships, and family life difficult.

4. Shift Work Sleep Disorder

Any working hours outside of the typical 9:00 a.m. to 5:00 p.m. schedule is considered shift work. While this could be something like 11:00 a.m. to 7:00 p.m., it’s often characterized as the “graveyard shift,” in which people are working overnight.

As you can imagine, this sort of schedule has a big impact on your circadian rhythm because you’re sleeping during daylight hours when your body naturally receives its wakefulness signals and working through the night when your body naturally wants to sleep. As a result, the health problems11 reported among shift workers are numerous and can include ulcers, heart disease, and pregnancy issues.

Scientists are actively working on ways to make this less of a detriment to our health, but in the meantime, if you must live with this type of schedule, try to maintain the same schedule, even on your days off.

5. Jet Lag

Flying long distances through multiple time zones can wreak havoc on your body’s internal clock. Even a difference of just a couple of hours is enough to throw your schedule completely off.

Depending on which direction you’re traveling, you may be extra tired or not tired at all when you arrive at your destination. Then, when morning comes, you’re either going to be up early or struggling to get up on time.

Symptoms12 don’t stop with sleep issues either. Jet lag can cause daytime fatigue, an inability to focus, digestion issues, a general feeling of being unwell or sick, and changes in mood.
Generally, it is thought that traveling east might make jet lag more severe than traveling west, but either way, there are some things you can do to prevent jet lag from being too severe:

  • Arrive to your destination early
  • Get plenty of sleep before your trip
  • Stay hydrated as dehydration can make jet lag symptoms worse
  • Gradually adjust your sleep schedule before your trip
  • If it is nighttime at your destination, try to sleep on the plane
  • If it is daytime at your destination, try to stay awake12

How To Maintain a Healthy Circadian Rhythm

Maintaining a Healthy Circadian Cycle

The easiest way to keep your circadian rhythm on track is to practice good sleep hygiene and maintain a regular schedule for sleeping, including on weekends. This won’t necessarily be easy –it will take dedication and practice – but here are some tips to make it a little bit simpler:

  • Get enough sleep: In general, we require seven to nine hours of sleep per night. Figure out what your ideal number is and plan to get that amount each night by counting those hours backward from the time you’ll have to wake up in the morning. For example, if you have to wake up at 7:00 a.m. and you know that you work best with eight hours of sleep, you’ll need to prioritize going to bed by 11:00 p.m. at the latest.
  • Maintain regular sleep hours: Maintain this same sleep schedule (11:00 p.m. to 7:00 a.m., for example) on weekdays and weekends. This consistency is important as it trains your body to get back into its regular circadian rhythm.
  • Avoid late afternoon naps: If you’re not getting enough sleep at night, you’re more likely to take a nap in the late afternoon or evening. Unfortunately, this will further throw your sleep schedule off, so if you need to nap, it’s best to do so early in the day.
  • Avoid electronics starting up to 2-3 hours before bedtime: Blue light from your phone and television will disrupt melatonin production13. Melatonin is the hormone that induces sleep, so if you don’t have enough of it in your system, it’s going to take you longer to fall asleep.
  • If 2-3 hours seems unreasonable for you, try to give yourself at least 1 hour before bed without screens.
  • Limit caffeine: That post-lunch cup of coffee could be messing with your circadian rhythm. Caffeine14 can stay in your system for up to 9.5 hours, and although it won’t be as strong as when you take the first sip, you should plan to stop drinking caffeinated drinks by noon each day. If you really need that almond milk latte to get you through the afternoon, consider half-caf.
  • Limit alcohol: Even though alcohol might make you feel drowsy, studies15 show that it actually disrupts the quality of sleep you’re getting. In fact, in any amount, alcohol delays16 the initial onset of REM sleep and reduces the total amount of time you spend in REM sleep through the night.
  • Take a morning walk: Light is the most powerful external trigger to sync our circadian rhythm. The combination of sunshine, fresh air, and movement is a powerful combination to wake you up and energize you for the day ahead.
  • Keep a dark sleeping environment: As we mentioned earlier, light disrupts melatonin production. Even turning on the bathroom light in the middle of the night can throw off your circadian rhythm. If you live in an area where there are bright street lights or lights from surrounding buildings, make sure you draw your shades, or better still, invest in some blackout curtains or an eye mask.
  • Exercise regularly: Exercise is vital for physical and mental health, but did you know it’s important for sleep as well? Research17 shows that regular exercise improves sleep quality and duration. In terms of getting your circadian rhythm back in order, working out in the morning sunshine would be ideal (combining the benefits of early morning light and exercise), but if that’s not possible, just pick the time of day that works best for you.


We often hear terms thrown around like “biological clock,” “circadian rhythm,” and “sleep-wake cycle” without truly grasping what they mean or how much they affect us all. As beings on this planet, we all have a circadian rhythm, governed by various “biological clocks” within our bodies, executing various functions like “sleep,” “wake up,” “eat,” etc. When these systems get out of whack, though, it can lead to disruptions in our circadian rhythm. Fortunately, there are things we can do to help get back on track to function to the best of our ability.

Natalie Grigson

Natalie Grigson


About Author

Natalie is a content writer for Sleep Advisor with a deep passion for all things health and a fascination with the mysterious activity that is sleep. Outside of writing about sleep, she is a bestselling author, improviser, and creative writing teacher based out of Austin.

Combination Sleeper


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