What is a Circadian Rhythm and How does it Work?

August 22, 2022 - Parul Saini, Webmedy Team

Updated Version - July 12, 2023

Circadian rhythm affects your sleep pattern and the functioning of your body. It affects your hormones, body temperature, and eating habits. When the circadian rhythm gets out of sync, it causes several health problems including diabetes, obesity, and depression.


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In this article, we will describe what is a circadian rhythm and how it works.

What is a Circadian Rhythm?

A circadian rhythm or circadian cycle, is a natural internal clock that regulates the sleep and wake cycle. It repeats roughly every 24 hours.

Different systems of the body follow circadian rhythms that are synchronized with a master clock in the brain. This master clock is directly influenced by environmental cues, especially light, which is why circadian rhythms are tied to the cycle of day and night.

It has been found that circadian rhythm can also be influenced by other factors such as ambient temperature, meal times, stress and exercise.

Let us look at how a circadian clock affects the human body at different times.

Effects of Circadian Clock times on Human Body

  • Best coordination at 2:30pm afternoon
  • Fastest reaction time at 3:30pm afternoon
  • Greatest cardiovascular efficiency and muscle strength at 5 pm afternoon
  • Highest blood pressure at 6:30pm evening
  • Highest body temperature at 7pm evening
  • Melatonin secretion starts at 9pm night
  • Bowel movements suppressed at 10:30pm night
  • Deepest Sleep at 2am night
  • Lowest body temperature at 4:30am morning
  • Sharpest rise in blood pressure at 6:45am morning
  • Melatonin secretion stops at 7:30am morning
  • Bowel Movement likely at 8:30am morning
  • Highest testosterone secretion at 9am morning
  • High alertness at 10am morning

How does a Circadian Rhythm work?

The master clock is found in the suprachiasmatic nucleus or S.C.N, which is in a part of the brain called the hypothalamus.

The suprachiasmatic nucleus (SCN) is responsible for regulating circadian rhythms in all organs. It receives direct inputs from a class of nerve cells in the retina that act as brightness detectors, which can reset the clock genes in the SCN on a daily basis. The SCN then transmits to the rest of the brain and body signals that bring all of the daily cycles in synchrony with the external day-night cycle.

When circadian rhythm is properly synchronized, it can promote consistent and restorative sleep. But when this circadian rhythm is not in sync, it can create problems in diverse aspects of physical and mental health, including sleep problems.

The suprachiasmatic nucleus has a direct neural connection to the pineal gland, which releases the hormone melatonin at night. When Pineal gland in brain is stimulated from darkness at night, it makes melatonin which is circulated in the blood. Melatonin helps in cleaning up free radical damage and oxidative stress in the body. Your exposure to sun light during the day can boost melatonin levels at night during sleep.

Helpful Information

What is a circadian rhythm?

Circadian rhythm refers to the physical, mental, and behavioral changes in living organisms that follow a daily cycle. These natural processes occur roughly over a 24-hour period and are driven by an internal biological clock. They influence sleep-wake cycles, hormone release, eating habits, digestion, body temperature, and other important bodily functions.

How does the circadian rhythm work?

The circadian rhythm is regulated by an area of the brain known as the suprachiasmatic nucleus (SCN), located in the hypothalamus. The SCN receives information about light exposure directly from the eyes and adjusts the body's internal clock accordingly. This internal clock then sends signals to various parts of the body to regulate activities such as sleep, metabolism, and hormone production.

Why is the circadian rhythm important for our health?

The circadian rhythm is vital because it regulates many essential functions of our body, including sleep, feeding, hormone production, and cell regeneration. It helps us align these bodily functions with the day-night cycle. Disruptions in the circadian rhythm can lead to sleep disorders, mental health disorders, metabolic disorders, and other health problems.

How does light exposure affect the circadian rhythm?

Light exposure, particularly natural light, is a crucial external cue for regulating circadian rhythms. The light detected by our eyes is relayed to the SCN, which uses this information to adjust our internal clock. Morning light exposure promotes wakefulness and sets the rhythm for the day, while the decrease in light in the evening prompts the body to prepare for sleep.

Can circadian rhythms be altered or adjusted?

Yes, circadian rhythms can be altered or adjusted, often in response to external cues, a process known as entrainment. Shifts in light exposure, such as those caused by travel across time zones or changing shift work, can reset our biological clocks. However, frequent significant alterations can lead to circadian rhythm disorders and associated health issues.

What is circadian rhythm sleep disorder?

Circadian rhythm sleep disorders are a family of sleep disorders characterized by disruptions in the timing of sleep, leading to insomnia, excessive daytime sleepiness, or both. They can be caused by a mismatch between a person's internal circadian rhythm and the external environment (as in shift work disorder or jet lag disorder) or by an abnormality in the internal clock itself (as in delayed sleep phase disorder or advanced sleep phase disorder).

How does the circadian rhythm affect sleep?

The circadian rhythm plays a significant role in regulating our sleep-wake cycle. It controls the production of melatonin, a hormone that promotes sleep, which rises in the evening (prompting us to feel sleepy) and falls in the morning. Disruptions to this rhythm can lead to trouble falling asleep, staying asleep, or waking up.

What factors can disrupt our circadian rhythms?

Various factors can disrupt circadian rhythms. These include exposure to light during the night (including the use of electronic devices), shift work, travel across time zones (leading to jet lag), certain medications, and lifestyle factors such as irregular sleeping and eating patterns. Aging and certain health conditions may also disrupt the circadian rhythm.

How does the circadian rhythm affect metabolism?

The circadian rhythm helps regulate various aspects of metabolism. It controls the daily fluctuations in the release of metabolic hormones, hunger and satiety cues, digestion efficiency, and energy expenditure. Misalignment of circadian rhythms can lead to metabolic disorders such as obesity and type 2 diabetes.

How can I reset my circadian rhythm?

Resetting your circadian rhythm involves re-establishing a regular sleep schedule and exposing yourself to natural light during the day, while limiting light exposure at night. Regular physical activity, consistent meal times, and reducing caffeine and alcohol can also help. In some cases, clinicians may recommend light therapy, melatonin, or other treatments.

How does shift work affect circadian rhythms?

Shift work, especially night shift work, can disrupt circadian rhythms by forcing people to be awake and active during the night and sleep during the day, which is contrary to the body's natural cycle. Over time, this can lead to chronic misalignment between the internal circadian rhythm and the external environment, increasing the risk of sleep disorders, metabolic disorders, and other health problems.

How does jet lag affect the circadian rhythm?

Jet lag occurs when you travel across multiple time zones and your circadian rhythm becomes out of sync with the local time. This misalignment can lead to symptoms such as insomnia, daytime sleepiness, difficulty concentrating, and digestive problems. The body typically adjusts at a rate of about one time zone per day.

How is the circadian rhythm controlled in the body?

The circadian rhythm is controlled by the biological clock located in the SCN in the hypothalamus of the brain. The SCN receives signals about light levels from the eyes and adjusts the body's internal clock. It then sends signals to regulate bodily functions according to this rhythm, including sleep-wake cycles, body temperature, hormone release, and other physiological activities.

Can circadian rhythm disruptions lead to health problems?

Yes, disruptions in the circadian rhythm can lead to a range of health problems. These include sleep disorders, mental health conditions such as depression and bipolar disorder, metabolic disorders like obesity and diabetes, cardiovascular diseases, and even an increased risk for certain types of cancer.

What role does diet play in the circadian rhythm?

Diet plays a significant role in the circadian rhythm. The timing of meals can influence our internal body clock. Consistent meal times can help regulate the circadian rhythm, while erratic eating patterns may disrupt it. Certain nutrients also seem to have a cyclical impact on the body and may interact with the circadian rhythm.

How can I tell if my circadian rhythm is off?

Signs that your circadian rhythm may be off include difficulty falling asleep or waking up, feeling tired during the day, and being awake and alert late at night. Other symptoms can include changes in appetite or digestion, mood changes, and difficulty focusing or remembering.

How does exercise impact the circadian rhythm?

Regular physical activity can help regulate the circadian rhythm. Exercise can act as a time cue for the body, and exercising at consistent times each day can help reinforce your daily rhythm. However, intense exercise close to bedtime might interfere with sleep.

What are some strategies for managing a healthy circadian rhythm?

Maintaining a regular sleep schedule, getting exposure to natural light during the day, limiting exposure to artificial light in the evening, regular physical activity, and consistent meal times can all help manage a healthy circadian rhythm. It's also important to manage stress and avoid stimulants like caffeine and alcohol close to bedtime.

What is the relationship between the circadian rhythm and mental health?

Circadian rhythm disruptions have been linked to several mental health conditions, including depression, bipolar disorder, and anxiety disorders. This relationship appears to be bidirectional: mental health conditions can disrupt circadian rhythms, and disruptions to circadian rhythms can exacerbate mental health conditions. Regular sleep-wake cycles are often a key component of mental health treatment plans.


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