What Is Circadian Rhythm?

Medically reviewed by
 Dr. Jade Wu, PhD, DBSM

Dr. Jade Wu, PhD, DBSM

Dr. Jade Wu, Ph.D. is a licensed clinical psychologist specializing in behavioral sleep medicine. She completed her Ph.D. at Boston University, and finished her medical psychology residency and clinical fellowship at Duke University School of Medicine.

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Last Updated On December 13th, 2023
What Is Circadian Rhythm?

Key Takeaways

  • What Are Circadian Rhythms: Circadian rhythms are biological cycles that run approximately every 24 hours and regulate essential functions like sleep, metabolism, and hormone release. These rhythms are present in most living organisms and are influenced by external cues, primarily light and ingestion.
  • Where Do Circadian Rhythms Come From: The master clock of our circadian rhythm is located in the suprachiasmatic nucleus (SCN) in the brain, which coordinates the production of circadian rhythms in various organs and tissues throughout the body. Light exposure to the retina plays a crucial role in synchronizing our internal circadian clock with the external day-night cycle.
  • How Circadian Rhythms Are Disrupted: Disruptions to circadian rhythms can lead to various health issues, including sleep disorders, mood disorders, and metabolic disturbances. Maintaining a regular sleep schedule, limiting exposure to bright light in the late evening, and avoiding caffeine at night are essential strategies for promoting a healthy circadian rhythm and overall well-being.

Circadian rhythm is a biological process grounded in time. The term circadian originates from the Latin phrase “circa,” meaning “about” and “diem,” meaning “a day”. Therefore, circadian rhythm roughly translates to “around a day’s rhythm.”

This is because human circadian rhythms runs approximately every 24 hours, regulating metabolism, hormone release, body temperature, and of course, the sleep-wake cycle. Some people simplify circadian rhythm by referring to it as our body’s internal clock. It’s directed by a master clock located in our brain. But does our brain actually contain a clock? Well, kind of.

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Let’s take a closer look at what circadian rhythm is, how it works, and why it is important to sleep health.

How Does Circadian Rhythm Work?

As mentioned above, circadian rhythms are predictable, biological cycles that coordinate essential mental and physical functions, such as sleep and hunger. The rhythms are present in most living organisms, including humans, plants, animals, and even fungi. In fact, most chronobiology (the study of circadian rhythms) researchers track the cyclic patterns of fruit flies and mice to understand this biological process.

These studies by the National Library of Medicine (NIH), Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source have found that circadian rhythms are endogenously generated, meaning they originate from within the body. This is because our internal master clock manages the production of the body’s circadian rhythms.

The Master Clock and Biological Clocks

Our brain contains a master clock called the suprachiasmatic nucleus (SCN), sometimes called the circadian pacemaker. As the master clock, our SCN directs other biological clocks Verified Source Oxford Academic Research journal published by Oxford University. View source located in peripheral tissues.

Nearly every organ and tissue within our body contains a biological clock made from genes and proteins; the most crucial proteins being CLK (clock) and PER (period). CLK and PER proteins have a seesaw effect because they enable and disable one another, keeping each other in a cyclic balance. This seesaw cycle helps to keep clocks on time. 

However, our external environment affects circadian rhythms as well. Cues from light, ingestion, temperature, noise, and physical activity maintain or disrupt the 24-hour cycles. These external cues are called Zeitgebers—German for “givers of time.” Light and ingestion are the most impactful Zeitgebers.

How Does Light Affect Circadian Rhythm?

Our master clock, or SCN, is composed of 20,000 nerve cells Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source located directly above the optic chiasm—optic nerves connecting the brain and eyes. The SCN also extends toward the center of the brain to the pineal gland, a pea-sized gland best known for producing the sleep-promoting hormone, melatonin. Why does all of this matter? The SCN receives messages from the retinas in our eyes in order to tell time and then passes the time-of-day information to the pineal gland to help it produce melatonin at the right times.

The retina is a layer of the eye sensitive to light. The structure contains photosensitive cells that help the SCN to sync our internal circadian rhythms with the light of the day and darkness of night. When light reaches the retina, its photosensitive cells send a signal to the SCN, which in turn signals the pineal gland to stop producing melatonin.  When it’s dark, the retinal photosensitive cells don’t send any signals to the SCN, so the pineal gland freely releases melatonin, making us sleepy.

Together, our retinas, SCN, and pineal gland naturally align our sleep-wake cycle with day and night. However, indoor light affects our internal body clock as well.

This explains why watching a lot of television, playing video games, or scrolling on our phones before bed may delay sleep onset.

See also Cellphone Use Statistics by State

How Does Ingestion Affect Circadian Rhythm?

Since your bodily functions give feedback to the SCN, eating at an appropriate time will help the SCN to maintain your daily rhythms. But what is the best time to eat? Active bodily functions are aligned with daylight hours. Therefore, specialists from the NIH recommend Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source eating with the sun as it will optimize your metabolic processes—chemical reactions in our cells that balance energy. That means eating in the morning, afternoon, and early evening.

When we eat, our pancreas releases insulin. Insulin is a hormone that allows glucose (sugar) to enter our body’s cells. Our cells then transform the glucose into energy. During the day, we are insulin sensitive, meaning our body uses glucose more effectively than at night. At night time, our body’s become insulin resistant, making it difficult for our cells to use glucose for energy. Instead, it is stored as fat.

What Disrupts Circadian Rhythm?

Our biological clocks innately align our physical, mental, and behavioral processes with day and night. For instance, we are biologically wired to be productive during the day but rest at night.

Disruptions to our day-night cycle can throw off our entire circadian rhythm, negatively affecting our coordination, energy, digestive system, sleep patterns, and much more. So how do you know if your circadian rhythm is off? The most obvious sign of a disrupted circadian rhythm is lethargy, but you may also experience the following:

  • Sleepiness during the day
  • Irritability
  • Clouded thinking and difficulty remembering things
  • Trouble socializing
  • Depression, anxiety, or negative moods
  • Weight gain
  • Gastrointestinal issues
  • Lack of coordination
  • Less motivation and energy
  • Tension headaches

Circadian Rhythm Disorders

People with circadian rhythm sleep disorders struggle with their sleep schedule. They find it troublesome to fall asleep, stay asleep, and wake at the desired times. There are six unique circadian rhythm sleep disorders. We will discuss the causes and symptoms of each.

1. Delayed Sleep Phase Syndrome

Delayed sleep phase syndrome is when someone’s sleep-wake cycle is two or more hours delayed compared to the conventional timing (e.g., they may not get sleepy until 2 a.m. or even later). These people are typically known as night owls and late sleepers. The syndrome is common in teenagers and may be a side effect of puberty.

Signs of delayed sleep phase syndrome include increased productivity at night and excessive daytime sleepiness if forced to wake up in the early morning

“Delayed sleep phase is not necessarily unhealthy,” explains Dr. Jade Wu, behavioral sleep medicine researcher. “If someone with a delayed sleep phase gets an adequate amount of sleep and is allowed to sleep at their preferred times, they will be just as healthy and productive as morning people. But they may struggle with physical or mental health problems if they consistently have to get up earlier than their preferred wake time.”

2. Advanced Sleep Phase Syndrome

People with advanced sleep phase syndrome fall asleep and wake up several hours earlier than the conventional sleep-wake phase. For instance, they may begin feeling sleepy as early as 6 p.m.

Advanced sleep phase syndrome does not negatively impact sleep quality but may affect one’s ability to fulfill their daily responsibilities. Sleep specialists are unsure of the cause, but aging may play a role.

3. Irregular Sleep-Wake Syndrome

Irregular sleep-wake syndrome is when someone does not have a consolidated nighttime sleep schedule. Regardless of the time of day or night, they may fall asleep for a minutes or a few hours, with no discernible pattern from day to day. The syndrome is rare and typically occurs in people with a neurodegenerative condition, like Alzheimer’s disease. Traumatic brain injury is also a common cause, as those who try sleeping after a concussion find.

Symptoms include napping for prolonged periods during the day as well as difficulty falling and staying asleep at night. A person is diagnosed with this syndrome if they experience three or more abnormal sleep-wake cycles within a single day.

4. Non-24 Hour Sleep Wake Disorder

People with non-24 hour sleep-wake disorder cannot keep their sleep-wake on a 24-hour schedule. Their sleep and wake time progressively delays by minutes, and sometimes hours, until it rotates all the way around the clock. For instance, a person feel sleepy at 12 a.m. bedtime tonight, 1 a.m. tomorrow, and 2 a.m. the night after. 

Most people suffering from the disorder have a regular day and night time schedule, so it severely disrupts their daily routines. The disorder is most common amongst blind people because their SCNs do not receive light-induced signals from their eyes, so their central clocks cannot adapt to the 24-hour light-dark environment.

5. Shift Work Disorder

What Is Circadian Rhythm?

Shift work disorder is common amongst people that work graveyard, swing, early morning, or rotating shifts due to irregular sleep-wake timing, meal times, and exposure to light. Common symptoms are insomnia, lethargy, excessive sleepiness, and lack of restorative sleep.

6. Jet Lag Disorder

Crossing multiple time zones during travel exposes us to external stimuli different than we are used to. This throws off our circadian rhythm, affecting our sleeping and eating habits. Jet lag symptoms include difficulty sleeping, fatigue, trouble concentrating, irritability, and gastrointestinal issues. Most people recover from jet lag within a few days, but frequent travel makes maintaining circadian rhythm difficult.

How to Maintain Circadian Rhythm

Disrupting the circadian rhythm is easy, but luckily you can reset it with some discipline. Let’s go over a few ways you can regulate your sleep patterns.

  • Go for a morning walk. Morning sunlight helps us to reset our circadian clocks. If you’ve had a recent change in schedule or have difficulties sleeping, consider a morning walking routine to reap the benefits of morning sunlight. Your circadian rhythm will eventually familiarize itself with the energy increase during your morning walk and begin promoting alertness at the same time each day.
  • Establish a sleep schedule. Having a set rise time may seem elementary but is a crucial part of your circadian rhythm. If you sleep and wake at about the same time each day, your master clock will quickly adapt and promote sleepiness or alertness at those specific times, ensuring better sleep.
  • Limit your exposure to bright light in the late evening. As mentioned above, light suppresses melatonin production. According to the Centers for Disease Control and Prevention, we are most sensitive to light two hours before bed. Staring at a television, computer, or phone screen within this time frame delays sleep onset (falling asleep). Consider adjusting the warmth of your screens or avoiding them altogether.
  • Limit your naps. Avoid napping longer than 30 minutes or near bedtime, as this will decrease your sleep drive and disrupt normal sleep patterns.
  • Avoid caffeine in the evening hours. Caffeine increases your autonomic arousal, which makes it harder to feel your natural sleepy cues and throw off your sleep-wake rhythm. 
  • Adjust the temperature of your bedroom. Our SCN also regulates body temperature to promote sleepiness and alertness. This process is called thermoregulation. Our body temperature decreases naturally as we transition to sleep and increases in preparation for being awake. Therefore, if your bedroom is too warm, you may struggle to cool off and find rest. Keeping your room temperature to a comfortably cool level will encourage thermoregulation.


Are biological clocks the same as our circadian rhythm?

Biological clocks and circadian rhythms are not the same. “Biological clocks” refer to certain proteins and genes found in nearly every cell in the body. These proteins and genes interact with one another to produce circadian rhythms.

Are circadian rhythm innate or learned?

Circadian rhythm are both innate and learned. They are innate because specific proteins, genes, and neurons within our body are programmed to run in a cyclic pattern. However, it can also be learned and even manipulated. External stimuli, like light and food, impact our daily rhythms. Therefore, exposing yourself to light or eating within a specific time frame will result in more robust rhythms and better sleep.

What happens to circadian rhythms when there is no sunlight?

As mentioned above, our master clock is highly sensitive to sunlight. Sunlight promotes alertness and can reset a disrupted circadian clock. A lack of sunlight, such as during the winter in the very far North, may result in lethargy in the day and energy at night. Artificial light may ease the effects of limited sunlight, but it does not emit nearly as much energy as the sun’s rays.

Does daylight savings time disrupt circadian rhythm?

Daylight savings time may force you to fall asleep, wake up, eat, and exercise at different times than you are used to. This can temporarily throw off your circadian rhythm and disrupt your bodily functions. You can prepare for daylight savings by gradually advancing or delaying your routine by a few minutes each day. For instance, if the time change will be an hour ahead, begin going to bed and waking up a few minutes earlier each night, until you reach an hour earlier.

Does circadian rhythm affect mental health?

A study published in The Lancet Psychiatry measured the activity levels of 91,000 participants with a small wrist monitor. The participants more active at night than during the day were more likely to develop mood disorders, such as depression or bipolar disorder. They also experienced subjective feelings of well-being. This study suggests unnatural circadian rhythms develop or exacerbate mental health issues.

Does age affect circadian rhythm?

Circadian rhythms can change with age. For instance, in utero, a fetus’ circadian rhythm is tied to their mother’s hormone levels. Once they are born, they take 3 to 4 months to adapt to an unfamiliar environment and thus, develop their own circadian rhythm. This explains newborn babies’ erratic sleep patterns.

Adolescents also experience circadian disruptions. Puberty causes circadian rhythms to shift later, making teens into natural night owls. This is why many teenagers have a delayed sleep phase disorder, meaning they fall asleep and wake up later than usual.

Young adults and middle-aged adults with set schedules have relatively normal circadian rhythms. However, many older adults may encounter circadian changes sleep disruptions. This is because older adults have less sensitivity to light and less robust master clock signals.


We all live our lives by rhythms and patterns, but the circadian rhythm is one of the most important, and often the most neglected. Sleeping in line with your natural circadian rhythm will lead to deeper rest and faster recovery. You’ll wake up feeling recharged and be more productive during the day. Be sure to dim the lights when the sun goes down, and get plenty of bright light during the day. Your master circadian clock will thank you and help you feel more alert during the day, more sleepy at night, and generally more well-rested. 

About the author

Stacy Liman is a journalism graduate student and a freelance writer with a focus on mindfulness and content marketing. Her passion for understanding and writing about the science of sleep enables her to provide valuable insights into achieving healthier and deeper rest. Stacy's commitment to helping people improve their sleep drives her exploration of new mattresses and sleep-promoting gadgets.

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