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Everything You Need to Know About Circadian Rhythm and its Associative Role with the Sleep-Wake Cycle

In this globalized world with rapidly revolutionizing industries, most people work more than 8-9 hours daily. Millennials have to understand the importance of clocks. Here, the clock doesn’t mean your wristwatch but your internal biological clock. 

The biological clocks housed within the body are necessary for every systemic function. In fact, plants, animals, and even microbes (fungi, cyanobacteria) also possess these biological clocks. 

Among the four biological clocks, the circadian rhythm becomes the most popular. The circadian rhythm controls your body’s 24-hour cycle-dependent physical, behavioral, and mental processes. 

The most important function of circadian rhythm is to regulate sleep. Yes, the 24-hour sleep-wake cycle of an individual is controlled by this biological clock. Now let’s explore how circadian rhythms are associated with sleep and sleep disorders in the latter part of the blog.

Biological Rhythms: An Introduction

As previously stated, biological clocks or biological rhythms are those natural and in-house systems found within your body to regulate the various cycles, metabolisms, functions, and biological processes. The biological clock cells prevail throughout the body, produce a certain set of proteins or molecules, and signal the central nervous system and the brain. Almost all cells, tissues, and organs contain biological clock cells.

Circadian Rhythm: An Overview

Circadian rhythms, the internal biological clock, runs throughout the day in the background, say a 2020 Sleep Foundation article. The term circadian originates from the Latin word “Circa Diem,” which means “around a day.” The circadian rhythms are endogenously produced as input signals come from the outside environment (light or temperature).

Apart from the sleep-wake cycle, the circadian rhythm controls the food intake patterns, brain wave activity, synthesis and release of hormones, cell regeneration, and many other biological functions of the body.

Master Clock: The Maestro Regulator

The circadian rhythm is connected throughout the body via the master clock, the suprachiasmatic nucleus (SCN). The 20,000 neurons that make up the SCN are found in the brain’s hypothalamus region. This master clock controls the entire network of biological clock cells across different body parts, specifically the circadian clock cells. Hence, it is also referred to as the circadian pacemaker of the body. 

The light and dark environment is initially sensed by the receptor cells in the eyes and signaled to the SCN, which further regulates the circadian rhythm and other biological clocks to regulate the day-to-day body functions and metabolisms.

biological clock

Primary Functions of Circadian Rhythm

Sleep-wake cycle:

As the SCN receives respective light signals from the eye’s optic nerve fibers and regulates the secretion of melatonin hormone, the circadian rhythm plays a vital role in the onset of nocturnal sleep. As SCN controls cortisol hormone secretion, the wake in the morning also depends on the circadian rhythm.

Release of hormones:

When the eye senses less light (from the evening), the body gets set for the melatonin secretion directed by the SCN and the circadian rhythm. On the contrary, when your eye senses higher light (from early morning), the body prepares itself to secrete the cortisol hormone. Also, other hormones, including leptin, ghrelin, and human growth hormone, are closely associated with circadian rhythms.

Food intake and weight gain:

The circadian rhythm, when affecting sleep, stimulates unusual night appetites that lead to untimely food intake. This ends up in uncontrolled weight gain and leads to obesity on a long note.


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Body temperature:

As the night approaches, your body temperatures tend to drop and aid the body in initiating the sleep phase. When the cortisol hormones are secreted in the morning in response to the morning light, the body temperature increases. Again, in the evenings, the body temperature starts to lower. This cycle of temperature changes is regulated by the circadian rhythm associated with SCN and brain functions.   


Different metabolisms follow different biological clock cycles. Those metabolisms that follow the body’s 24-hour day-night cycle depend on the circadian clock for smooth functioning. 

Apart from these primary functions, the circadian rhythm also interferes in

Factors Influencing Circadian Rhythm

External Factors:

  • Light or harmful radiation from gadgets and electronic devices, like mobile phones, laptops, TV, tablet, etc., at night may mimic the daylight and send wrong signals to the SCN, which further affects the circadian pattern of their body.
  • Night shift workers who experience consistent light hours in the night may have an altered 24-hour circadian cycle. This directs the body against the natural day-night cycle and causes serious health problems over the years.
  • Frequent travelers experience different time zones and regular jet lags that alter the circadian pattern. 
  • Millennials with a modern lifestyle of sleeping and waking late tend to have an unhealthy circadian rhythm that hinders their education or job performance.
  • An unhealthy eating and sleeping pattern causes a change in the 24-hour circadian cycle.
  • Space travel is a rare case wherein astronauts experience rapid fluctuations in their day-night cycle. Further ahead, when they enter the space (void), the complete darkness may mimic the nighttime. This dramatically affects their circadian rhythm and patterns. However, astronauts undergo the required training to adapt to the space environment easily.

Internal Factors:

  • Rarely do individuals undergo gene-level modifications or mutations. These unexpected internal changes affect almost all biological clocks, including the circadian rhythm. In humans, the period and the cryptochrome genes account for the circadian rhythm.
  • Developing mental stress over time affects the brain’s ability to send the right signals. As a result, the SCN in the brain dysregulates circadian patterns.  
  • Sometimes, medications individuals take to treat other health concerns affect the biological clock and rhythms. 
  • Hormonal changes during pregnancy and at times of menopause in women may influence the circadian cycle. However, not all women experience such changes.

When these factors affect the natural day-night and healthy circadian rhythm, the probability of bodily complications increases. Individuals may suffer primarily from sleep disorders and develop chronic conditions such as blindness, depression, obesity, brain damage, and diabetes.

Circadian Rhythms at Different Ages


Circadian rhythms are not observed in newborn babies. Babies only sense the environmental changes after a few months and develop the circadian rhythm. Until then, babies exhibit unusual and strange sleeping patterns. After three months, once the babies develop the circadian rhythm, they start releasing the melatonin and cortisol hormones that regulate their sleep and wake activities.



Children of this age show circadian patterns that recommend about nine to ten hours of sleep.


Unlike toddlers, teenagers have the habit of going to bed late due to playful activities at night. This age group may experience slightly later melatonin release (between 10 and 11 pm as opposed to 8 to 9 pm). This change in the cycle affects their peak sleeping time, which happens only after 3am. Consequently, waking activity is also affected.


As grown-ups, adults have matured and completely developed circadian rhythms. Adults with healthy habits show a healthy and consistent circadian rhythm pattern. In contrast, adults with changing lifestyles, eating, and sleeping habits show poor circadian rhythm that may develop short and long-term effects on the body. A healthy sleeping schedule for adults is seven to nine hours a night.

Older people:

As people age, their circadian patterns change naturally. These people tend to fall asleep earlier in the night (early onset of melatonin secretion) and face difficulties waking up early in the morning (early onset of cortisol secretion).

Effects of Distorted Circadian Rhythm and Sleep

A Healthline article lists both short-term and long-term effects that your body experiences due to a distortion in the healthy circadian rhythm and sleep.

  • Short-term effects: Poor inflammatory response, lack of memory, fatigue, low energy throughout the day, poor sleep, and increased arousal at night. 
  • Long-term effects: Severe damage to organs and their functions, uncontrolled weight gain (obesity), sleep apnea, development of cardiovascular diseases, stressed brain functions, affected and dysregulated metabolisms, fluctuated sugar levels with higher chances of diabetes, gastrointestinal problems, and skin-related issues.

According to a research source, long-term circadian distortions may also lead to premature mortality, anxiety, cancer progression, impaired glucose tolerance, and psychiatric disorders. On the other hand, short-term distortions may lead to total impaired wellness and loss of concentration.

Circadian Rhythm and Sleep Disorders

Circadian rhythm-associated sleep disorders are on the rise due to poor sleep habits among millennial adults. Delayed sleep phase disorder is a circadian rhythm sleep disorder with serious ill effects. Teens and youngsters who sleep and wake up late have higher chances of encountering the delayed sleep phase disorder. Another disorder, the advanced sleep phase disorder, is when individuals sleep and wake up early like older people.

The most common symptoms of a circadian rhythm sleep disorder include the following:

  • Severe sleep loss
  • Depression 
  • Physical and mental stress
  • Signs of insomnia or sleep apnea 
  • Drowsiness
  • Low energy levels throughout the day


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Ways to Retain a Healthy Circadian Rhythm

Here are some tips to keep your sleep and the circadian clock healthy. Try them to avoid circadian dysregulation and associated sleep disorders.

  1. Follow a healthy lifestyle: A nutrition-rich diet and timely food intake make your circadian clock consistent in the day-night cycle.
  2. Get sunlight daily: Avoiding artificial lights from electronics and seeking natural sunlight helps align the circadian rhythm on a proper 24-hour cycle.
  3. Give up on caffeine intake: Daily caffeine intake increases the chances of nocturnal arousal and wakefulness. 
  4. Exercise daily: Slight exercises a couple of hours ahead of sleep time aids in the easy onset of melatonin secretion and effortless sleep initiation.
  5. Ensure a regular sleep schedule: This prevents nighttime arousal and daytime sleepiness.
  6. Favorable sleep environment: Switching off the lights, setting a comfortable bed, and turning off the gadgets help induce sleep.
  7. Avoid afternoon deep sleeps: Taking power naps or short naps in the noon helps balance your energy and promote efficiency. However, prolonged naps at noon may hinder the timely initiation of sleep at night.


Having a healthy diet with the required hours of sleep keeps your circadian rhythm healthy. Conversely, a healthy circadian rhythm regulates body weight and increases memory or brain functions. So, ensuring to keep a controlled 24-hour circadian cycle is quintessential.

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