The Complete Guide to Circadian Rhythm

Table of Contents

If you’ve ever felt energized or tired around the same time each day, you’ve experienced the circadian rhythm at work. What exactly is a circadian rhythm, and why does it matter?

Circadian rhythm is the internal clock that regulates the body’s sleep and wake cycles over a 24-hour time period. Because circadian rhythm is controlled by the part of the brain that responds to light, alertness tends to peak during the day and decline at night. (1)

Researchers have been studying the circadian rhythm since the 18th century, when scientist Jean-Jacques d’Ortous de Mairan observed a plant opening and closing its leaflets each day in complete darkness, revealing the existence of a biological clock. (2)

Two hundred years later, scientists discovered that genes inform circadian rhythms, and circadian rhythms synchronize to light-dark cycles. (3)(4) Recently, researchers isolated a gene that controls the daily biological rhythm while interacting with the 24 hour light/dark cycle, revealing how the clock system works on a molecular level. (5)

Essential though circadian rhythm is to sleep and wake cycles, its importance doesn’t end there. The circadian rhythm interacts with the brain and body to regulate numerous important physiological functions, from blood pressure to core temperature to metabolism.

Note: The content on Sleepopolis is meant to be informative in nature, but it shouldn’t be taken as medical advice, and it shouldn’t take the place of medical advice and supervision from a trained professional. If you feel you may be suffering from any sleep disorder or medical condition, please see your healthcare provider immediately.


Q: Where does the word circadian come from? A: Coined by a sleep scientist in 1959, circadian is a Latin term that fuses “circa,” which means around, with “diem,” meaning day.

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How It Works: The Master Clock, Body Clock, And Circadian Rhythm

The circadian rhythm not only controls sleep and wake cycles, but impacts many key aspects of the lives of most organisms. (6Circadian rhythms are the reason travelers experience jet lag, certain marine species burrow into the sand instead of being swept out to sea, and birds migrate across long distances without losing their way. (7) (8)

The circadian rhythm is controlled by two main processes: the internal biological clock system and external surroundings. (9)

Each of the body’s biological clocks is governed by the “master clock,” a group of approximately 20,000 nerve cells that form the suprachiasmatic nucleus, or SCN. (10) The SCN, located in the hypothalamus in the brain, uses cues such as daylight to signal feelings of alertness and sleepiness. The SCN also contains receptors for melatonin, commonly termed “the sleep hormone.” Neurons in the SCN fire in a 24-hour rhythm, reaching a peak at mid-day.

In addition to sleep, the SCN regulates metabolism and hormone production. Through a complex process of hormone secretion and changes in body temperature, the SCN synchronizes “local” clocks that operate independently of the master clock. These gene-operated clocks are found in places such as the liver, connective tissue, lungs, and muscles. (11) Neuroscientists suspect that almost every cell in the body contains a circadian clock. (12)

Once the master clock releases signals that trigger certain body functions, the biological clock sets the circadian rhythm in motion. The biological clock produces daily circadian rhythms, monitors their timing, and controls other cycles that occur seasonally and yearly, such as a woman feeling that her “clock is ticking” near the end of her childbearing years. (13) The biological clock controls these cycles while coordinating with the master clock to keep the circadian rhythm in sync with the surrounding environment.

Suprachiasmatic nucleus, or SCN

The SCN is located in the hypothalamus region of the brain and uses signals from the eyes to produce circadian rhythms.

Examples of environmental cues that impact the circadian rhythm include:

  • Daily changes in light when the sun rises and sets
  • Temperature
  • Humidity
  • Activity levels
  • Meal times

In addition to producing circadian rhythms through their interaction with the day/night cycle and internal clock, these environmental cues help stimulate essential functions in the body, such as digestion, body temperature, and melatonin release. (14) (15)

Circadian Rhythm And Sleep

Circadian rhythms cue sleep and wake times in part through detection of light signals. Light enters the eye through the retina and travels down a neural pathway to the SCN, triggering alertness and the wake state. When the sun sets and there is less light, the master clock signals the brain to produce melatonin, which triggers drowsiness and promotes sleep. (16)

In addition to light, other factors contribute to sleep and wake times. These factors include:

  • Sleep deprivation
  • The length of time one has been awake
  • Daylight saving time
  • Seasons

Because circadian rhythm never shuts off, its effects are felt at all hours.  Depending on age and lifestyle, natural circadian rhythm may fluctuate. For instance, sleep deprivation can amplify the natural dips and rises of the circadian rhythm, leading to exaggerated feelings of sleepiness and wakefulness. Regular sleep habits are more likely to produce consistent energy levels throughout the day, as well as lower levels of daytime sleepiness.

During a typical day, energy begins a slow climb on awakening and continues until early afternoon. Soon after, energy begins to decline, hitting a low around 3:00 pm. After the 3:00 pm lull, energy increases again, peaking around 6 pm. Alertness tends to wane for the rest of the evening and into the early hours of the morning, hitting its lowest point at 3:30 am.

Circadian Rhythm And Health

Research reveals that circadian rhythms have an important influence on energy and fitness levels, as well as the functioning of essential bodily processes. A healthy circadian rhythm is linked to longevity, reduced stress levels, and a strong metabolism.

One recent study from MIT found that a protein that protects against diseases of aging also plays a critical role in controlling circadian rhythms. (17) Scientists have also discovered a specific protein that works with the body’s circadian clock to regulate key metabolic functions. (18)

Irregular circadian rhythms have been linked to chronic health conditions such as sleep disorders, obesity, and depression. (19) Research reveals that night shift workers have an increased risk of obesity and diabetes, and may be at increased risk for accidents. Blue light emitted by screens on cell phones, computers, tablets, and televisions restrains the production of melatonin and is a major contributor to an irregular rhythm.

Over thirty-five medical conditions may be affected by the body’s internal clock, including:

  • Diabetes
  • Heart disease
  • Cognitive decline
  • Bipolar disorder
  • Elevated blood pressure
  • High glucose levels

Circadian Rhythm And Metabolism

The circadian cycle is strongly associated with the body’s metabolism. (20) While genetics influence the body’s ability to maintain energy balance through food and drink consumption, studies reveal that both circadian rhythm and environmental cues such as light also play a key role.

In the last decade, researchers discovered that circadian rhythms help to regulate energy levels in cells. This may help explain why a disrupted sleep pattern can increase hunger, which could lead to weight gain, obesity, and other metabolic problems.

One recent study that examined the connection between circadian rhythm and metabolic function in rats found that replacing the dark period of a rodent’s circadian cycle with dim light increased the rodents’ body mass. (21) Research on humans found that circadian misalignment caused decreased concentrations of leptin, a hormone that promotes the feeling of fullness. (22)

Scientists now believe that “resetting” the biological clock by adjusting what is eaten and when could prevent metabolic disorders while also promoting good health and extending life span. (23)


Q: What is the retina? A: A light-sensing membrane at the back of the eyeball that sends signals to the brain, influencing circadian rhythm.

Circadian Rhythm And Aging

Circadian rhythms don’t remain the same from birth to old age. A 2015 study found that clock-controlled genes fluctuate in the brain as people grow older. (24) Researchers examined brain samples from deceased people of various ages, and found that younger brains had a daily rhythm in their clock genes while the rhythm had been lost in many of the genes of older people. Additional research suggests that a healthy circadian rhythm may help guard against such age-related diseases as dementia, hypertension, and Parkinson’s disease. (25)

One study found that a gene called SIRT1 protects against diseases of aging and plays a key role in controlling circadian rhythms. The same researchers also found that circadian function decays with aging, but that boosting the SIRT1 gene levels in the brain could prevent this loss. A decline in SIRT1 function disrupts circadian control while mimicking what happens when people age. This finding is promising as scientists believe enhancing circadian function — like delivering SIRT1 activators in the body— could reduce age-related health issues and lead to improved sleep. (26)

Circadian Rhythm And Stress

Both the biological clock and external cues such as light and darkness are strongly linked to mental health. Seasonal Affective Disorder (SAD), a type of depression closely connected to the seasons, may be the best-known example of how circadian rhythm can affect mood. SAD typically begins and ends at the same times of the year, diminishing as daylight hours increase.

Signs and symptoms of SAD include:

      • Feeling depressed during late fall and winter months
      • Losing interest in activities, particularly during the winter
      • Decreased energy
      • Difficulty sleeping
      • Appetite or weight changes
      • Trouble concentrating
      • Feelings of hopelessness or guilt
      • Thoughts of death or suicide

Repeated psychological stress on the body can disturb the circadian rhythm. (27) Some neuroscientists believe that chronic stress may lead to the production of irregular levels of the steroid hormone cortisol, which is essential for proper functioning of the circadian rhythm as well as modulation of mood and motivation. Prolonged cortisol dysfunction can lead to problems like depression, weight gain, or Cushing’s syndrome,  a disease of excessive cortisol production. (28)

A disrupted circadian rhythm can cause mood disorders at any time of year. Scientists studying mood and circadian rhythm manipulated mouse genes to suppress those responsible for regulating the biological clock. They found that the mice were slower to escape from an uncomfortable situation, a sign of hopelessness. The mice also gained more weight than control mice, despite eating the same amount of food. The mice with disturbed rhythms spent less time in bright light, a sign of anxiety, and were more immobile than control mice. (29)

Metabolic function

The process by which the body converts food and drink into energy.

Eating, Exercise, And Circadian Rhythm

The body’s metabolism follows a 24-hour clock, with hormones, enzymes, and digestive systems primed for food in the morning and afternoon. Some studies show that people who snack shortly before bed can disrupt their circadian rhythm, which is wired to not feed the body with additional energy at night. (30)

Other research reveals that when food is consumed is more critical to weight loss than the amount of food eaten. One study compared the effect of eating a large breakfast versus a large dinner, and found that the group that ate a large breakfast lost more weight than the group that ate a big dinner. This may be because the dinner group ate when circulating insulin, which has its own circadian rhythm, was at its highest. (31)

exercise and circadian rhythm

Exercise impacts circadian rhythm, as well. Muscles have their own internal clocks and, due to circadian rhythm, function better during the day than at night. (32) Physical strength and core body temperature peak in the late afternoon and influence exercise performance. (33) This may mean that daytime exercise is more efficient and beneficial to health than working out at night or in the early morning. (34)

Movement helps to maintain a healthy circadian rhythm, while a lack of exercise may throw off the body clock. In a study on mice, researchers found that exercise affects daily healthy movement patterns more than age does. This may mean that exercise makes the body better able to judge when it should be moving and when it should be at rest. (35)


Q: What is bright light therapy? A: A circadian rhythm disorder treatment that mimics natural outdoor light and affects brain chemicals linked to mood and sleep.

How The Circadian Rhythm Gets Disrupted

Circadian rhythm functions best in conjunction with healthy sleep habits, such as going to bed and waking at the same time each day. Irregular sleep and wake times may disrupt the circadian rhythm, leading to health complications.

The circadian rhythm may be impacted by other sleep disruptors, such as:

      • Jet lag
      • Daylight saving time
      • Staying up all night
      • Exposing the brain to artificial light from a smartphone, tablet, or television at night
      • Exercising late at night

Naps are unlikely to disturb circadian rhythm if scheduled during a low point of alertness and completed by approximately 3:00 pm.

Chronic disruption of the circadian rhythm may lead to sleep disorders or other related health issues. While some circadian rhythm disorders are intrinsic, meaning a person’s body clock is off-kilter from the rest of society’s, others are circumstantial, or extrinsic, meaning a person’s circadian rhythm is out of sync with typical light/darkness patterns. (36)

Circadian Rhythm Disorders

Delayed Sleep Phase Syndrome (DSPS)

This type of disorder occurs when someone feels tired later in the night and goes to sleep at least two hours past a reasonable bedtime, which for adults is often said to be by midnight. However, they still require the recommended 7-9 hours of sleep, but due to work, early school start times, or other life demands, can’t rise later in the morning.

For instance, instead of falling asleep at 10:30 pm and waking at 6:30 am, someone will DSPS will fall asleep closer to 1:00 am and have a difficult time waking up in time for school. DSPS is most common among adolescents; as many as 16% of whom have the disorder. (37)

Some researchers believe Delayed Sleep Phase Syndrome may be an excessive reaction to the normal shift in the internal clocks that teens experience after reaching puberty. DSPS sufferers often feel extremely tired during the day, experience insomnia, and are at a greater risk for depression or other psychiatric problems, including behavioral issues. (38)

Advanced Sleep Phase Syndrome (ASPS)

Another disorder similar to DPSP is advanced sleep phase syndrome (ASPS), which causes sufferers to feel tired earlier at night and go to bed earlier than is typical. (39) ASPS sufferers tend to feel tired in the late afternoon and go to bed between 6:00 and 9:00 pm. Because bedtime is early, they also wake up early — usually between 2:00 and 5:00 am. Waking at such an early hour may make falling back to sleep difficult, leading to insomnia.

Unlike DSPS, ASPS occurs more often in older people. This is likely due to a natural loss in response to light as people age, especially among those with eye problems, such as cataracts. While the exact number ASPS sufferers is unknown, researchers believe it’s less than 1% of the population. People living with ASPS experience early morning insomnia and excessive daytime sleepiness, and also have a higher risk for depression. (40)

Circadian rhythm disorders

A family of sleep disorders affecting the timing of sleep, among other bodily processes.

Non-24-Hour Sleep-Wake Syndrome

People who suffer from Non-24-Hour Sleep-Wake Syndrome have a circadian rhythm that is out of sync. This results in a sleep cycle that is slightly longer than 24-hours, forcing later daily sleep and wake times. A Non-24 sufferer with a 24.5 hour clock might sleep 30 minutes later on the first day, one hour later the second day, and so on. The sleep-wake cycle will eventually realign with the 24-hour light-dark cycle, providing temporary relief and conventional sleep. The sleep cycle will continue to shift, however, creating repeated cycles of later sleep and wake times.

Non-24 Syndrome is most common in people who are blind, due to the retina’s inability to transmit natural light to the hypothalamus portion of the brain. (41) The disorder may affect as many as three-quarters of people with total vision loss. While someone with sight could develop Non-24, it is extremely rare and underinvestigated. (42)

Irregular Sleep-Wake Rhythm

circadian rhythm and stress

This rare circadian rhythm disorder impacts people with sleep-wake cycles that don’t fit within the usual 24-hour clock. Instead, sufferers nap for as many as 7-9 hours throughout each day. This may cause excessive daytime sleepiness due to a disrupted circadian rhythm as well as an unusual sleep pattern. (43)

Scientists do not know how many people suffer from this disorder, nor are they entirely sure how it forms. Some researchers believe it develops in people with a weak body clock, which may result from such neurological conditions as dementia and brain damage.

Shift-Work Sleep Disorder

Shift-work sleep disorder (SWSD) results from a conflict between a sufferer’s the circadian rhythm and work schedule. This schedule may run counter to the internal body clock, leading to insomnia, excessive daytime sleepiness, and increased risk for cardiovascular and gastrointestinal problems. (44) (45)

“Shift-work” is defined as working hours outside of the traditional 9:00 am to 5:00 pm. time frame, whether the early morning shift, the night shift, or rotating work schedules. Due to lack of alignment between the sleep/wake and dark/light cycles, SWSD sufferers may have difficulty falling asleep, staying asleep, or sleeping when tired.

While some shift workers may be able to adapt, those with rotating schedules may find adjustment difficult or impossible. Approximately 20% of workers in the U.S. are shift workers, and as many as 40% of them may suffer from SWSD. (46)


Q: How many people in the U.S. suffer from insomnia? A: Roughly 60 million Americans are affected by the sleep disorder each year.

Untreated circadian rhythm disorders and chronic sleep loss may lead to the development of certain diseases. Some of the diseases linked to circadian rhythm disorders include:

  • Obesity
  • Diabetes
  • Sleep disorders
  • Depression
  • Certain cancers
  • Dementia

Treatment Of Circadian Rhythm Disorders

Timed exposure to bright light may help reset the circadian clock in those suffering from severe circadian rhythm disorders. (47) A high-intensity light of approximately 10,000 lux is used in most cases, and exposure lasts from one to two hours. Treatment timing is crucial, and requires assistance from a sleep specialist who administers the light.

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For less severe disorders like DSPS, behavior therapy is commonly used. Such interventions include establishing regular sleep-wake times, avoiding naps, sticking to regular morning or afternoon exercise, and avoiding caffeine and bright light from screens a few hours before bedtime. Standardized administration of melatonin has also been shown to be an effective treatment for circadian rhythm sleep disorders, and is especially helpful for jet lag. (48)

Tips For A Healthy Circadian Rhythm

The following tips may help keep the circadian rhythm in sync with internal and external cues:

      • Stick to a consistent sleep schedule. Sleep scientists typically recommend going to bed and waking at the same time each day. This may help to keep the circadian rhythm in sync and increase energy once awake. (49)
      • Go for a morning walk. Since energy levels are closely tied to light, open up blackout shades first thing and take a morning stroll. The exposure to the sun will provide an energy boost while jumpstarting the circadian rhythm, administering enough sun exposure to signal to the brain it’s time to start the day. (50)
      • Limit evening technology use. Exposure to artificial, blue light from phones is wreaking havoc on good health. Imagine being in bed while the sun is shining straight inside; there’s no way one could fall asleep. While that example is drastic, screen light puts the rhythm out of sync and makes it harder for people to sleep well. (51)
      • Time meals. A growing body of research suggests the body functions optimally when eating patterns align with circadian rhythms. Studies say that eating earlier in the day and having a light meal around dinner time is optimal for metabolic health. (52)
      • Sleep in darkness. A pitch-black room is an important signal to the body that it is time for rest. (53) Make sure there are no ways to let light in, whether it’s an open curtain or a blinking light from an electronic. Sleep masks may also be helpful.
      • Reduce stress. Stress responses vary throughout the day and can disrupt the circadian clock. Practice meditation, journaling, breathing techniques, or therapy, as ways to reduce stress.

Circadian Rhythm and Chronotypes

Circadian rhythms vary from person to person, and are influenced by age, genetics, and environmental factors. Every organism has a specific chronotype, also referred to as a biological clock. Individual chronotypes are determined by genetic predisposition, as well as the circadian gene known as period circadian protein homolog 3, or PER3. (54)

Chronotype controls the circadian rhythm as well as sleep, eating, and exercise patterns. Each chronotype is considered a “circadian rhythm personality,” and helps distinguish early risers from night owls. (55)

There are four main chronotypes, each named after an animal:

  • Bears. Bears tend to sleep and wake according to the sun. They typically feel most energetic during the day, and fall asleep easily at night
  • Wolves (Night Owls). Wolves generally sleep and wake later than others. They are often most productive in the middle of the day and in the evening
  • Lions (Early Birds). Lions wake up early and are most productive in the morning. Due to early wake times and daytime energy expenditure, they are usually tired by evening
  • Dolphins. Dolphins are light sleepers who have difficulty following a regular sleep routine. They are most productive in the mid-morning to early afternoon

Once an individual’s chronotype is identified, a healthy and consistent routine can be established to cater to the body’s natural rhythm. This may mean maintaining consistent bed and wake times, and eating meals and exercising around the same time each day, including weekends. The circadian rhythm can become disrupted when sleep and meal times differ on the weekends, creating social jet lag. (56)

Blue light

Short-wave blue light is light emitted by smartphones, computers, and tablets. It affects the body like sunlight, and may make falling asleep difficult.

Last Word From Sleepopolis

Circadian rhythms are powerful biological cycles that impact nearly every aspect of health and wellness. Not only do circadian rhythms determine sleep patterns, but they also influence hormone release, eating habits and digestion, body temperature, and other important biological functions. Advances in chronobiology will only increase scientists’ understanding of these internal time-tellers, leading to treatments for circadian-related disorders like depression, Seasonal Affective Disorder (SAD), chronic diseases, and jet lag. (57)

A healthy circadian rhythm helps promote energy and well-being while lowering the risk of developing certain illnesses. The circadian rhythm works best in conjunction with healthy habits, so make sure to maintain consistent bed and wake times, limit exposure to artificial light from screens at night, and stick to a consistent eating and exercise routine every day.


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Laura Schwecherl

Laura is a journalist with nearly a decade of experience reporting and covering topics in the health, fitness, and wellness space. She is also a marketing consultant, where she works with impact-oriented startups to build marketing and editorial strategies. Outside of work, you can find her reading Murakami novels, writing amateur poetry, or trail running in her hometown, Boulder, Colorado.