Circadian Rhythm: Definition, Physical, Mental, and Behavioral Features

Everyone has an internal master clock (circadian rhythm) that regulates time at a molecular level. To define the circadian rhythm, it is the physical, mental, and behavioral changes that occur in the body within 24 hours. 

All life forms, including humans, animals, plants, and other organisms, have a circadian rhythm. Fine tuning the circadian rhythm is essential for life to develop. Plants know when to bloom, and animals know when to hibernate. During a sleep-wake cycle, the body can tell the difference between light and dark. 

Circadian derives from the Latin term Circa Diem, which means around the day. A circadian rhythm results from the biological clock, but it differs in all living beings.      

What are the Main Changes within the Circadian Rhythm Loop?

A healthy circadian rhythm regulates bodily functions over 24 hours. When the cycle malfunctions, the time frame can extend an extra hour. 

What Are the Main Changes within the Circadian Rhythm Loop
What are the Main Changes in the Circadian Rhythm Loop?

To learn how to fix circadian rhythm, think about what sets it off schedule. Anything can affect the circadian rhythm loop. Changes in the bedtime routine, lack of exercise, higher stress levels, and age can disorient the natural cycle.

Here are a few symptoms that may indicate a circadian rhythm sleep disorder. 

  1. Trouble concentrating 
  2. Difficulty remembering things
  3. Lack of motivation
  4. Trouble sleeping at night             

There are several ways to reset your circadian rhythm. One common technique is to follow a sleep schedule. Go to bed and wake up regularly every day to help your body adjust to its new rhythm. 

Shift workers may benefit from bright light therapy. Bright lights can help the brain get back on track and regulate circadian rhythms. Other ways of fine-tuning the circadian rhythm are taking melatonin supplements, adjusting your meal times, avoiding caffeine, and incorporating exercise into your day. 

Examples of the circadian rhythm include the sleep-wake cycle, body temperature, menstrual cycle, and respirations.

What Are the Physical Changes within the Circadian Rhythm?

The main physical response to the circadian rhythm is the sleep-wake cycle, which tells your body when to sleep and when to be awake. There are other ways your body can respond.

  1. Digestion
  2. Weight gain
  3. Infection control 
  4. Heart and circulation
  5. Other physiological functions 

What Are the Mental Changes within the Circadian Rhythm?

The circadian cycle affects your mood. If the rhythm becomes irregular, it can cause irritability, anxiety, or depression. The reason for those changes is most likely due to sleep deprivation.  

Circadian rhythm and memory are a concern for those with a sleeping disorder. A healthy functioning circadian rhythm refreshes memory cells. When the cycle is off, memory can suffer. 

What Are the Behavioral Changes within the Circadian Rhythm?

Disruption in the circadian clock can cause aggressive behavior. Lack of sleep and poor lighting transition are contributors to behavioral issues. Studies show that students who stay up late tend to have worse grades than those with regular sleep schedules.

What Are the Biological Clocks that Regulate Circadian Rhythm?

Endogenous oscillators are biological clocks, which are internal computers that dictate body functions. Every being has a type of biological clock. 

The five types of biological clocks or rhythms include diurnal (regulates night and day), circadian (works on a 24-hour schedule), ultradian (lasts less than 24 hours), infradian/ circalunar (run for a month), and Circannual, which is year-round. 

These stages of sleep are a part of the circadian cycle. 

  1. Quiet or Non-Rem Sleep. During this stage, body functions, such as temperature, muscle activity, heart rate, and breathing patterns decrease as you gradually drift off into a deep sleep. Progressively, it may become harder for you to awaken to stimuli. 
  2. Dreaming/REM Sleep. During this stage, rapid eye movement happens about every 90 minutes. Dreams may occur. Body functions accelerate to daytime levels, and your body prepares for a fight versus flight. 

How Long Does It Take to Complete a Circadian Rhythm?

According to a study through Harvard University, healthy adult humans undergo a circadian rhythm lasting about 24 hours and 11 to 16 minutes. Some circumstances can extend or shorten the cycle. 

The study also suggested that most people favored a 25-hour circadian rhythm after decreasing their external stimuli. Inconsistent sleep and wake-up times can also lengthen the cycle. Work shift changes, medications, and medical conditions can lead to longer reset times. 

Can you shorten the cycle? Yes. Light is a contributor to the quality of sleep you can get. Exposure to artificial light, such as interior lighting, can extend the cycle and make it more difficult for your body to adjust. 

Get more sunlight and decrease other stimuli to help you sleep better. Your cycle will be shorter when you are back on a regular sleep schedule.

Neurotransmitters are the chemical messengers of the body. Together, they work to help regulate the circadian cycle. 

What Are the Neurotransmitters Related to the Circadian Rhythm
What are the Neurotransmitters Related to the Circadian Rhythm?
  1. Acetylcholine (ACH) promotes healthy cognitive skills. 
  2. Serotonin regulates your mood. 
  3. Glutamate helps the brain normalize memory cells. Elevated levels can cause neurological disorders, such as ALS (Lou Gehrig) or Multiple Sclerosis.
  4. Neuropeptide Y (NPY) aids with digestion. 
  5. Vasoactive Intestinal Peptide (VIP) increases the contractility of the heart and decreases inflammation in the body.
  6. Peptide histidine isoleucine (PHI) acts as a type of hormone. 
  7. Arginine vasopressin (AVP) serves many functions in the body, such as regulating arterial blood pressure and balancing fluid levels. 

What Can Block Regulation of the Circadian Rhythm?

There are conditions where you cannot regulate the circadian rhythm. Though avoidable, they are common barriers to a healthy sleep cycle. 

  1. Artificial lights, such as lamps, overstimulate the brain. 
  2. Changes in eating patterns may make you feel bad, interrupting your sleep. 
  3. Illness can shift the timing of your biological clock and prevent restful sleep. 

Does caffeine affect circadian rhythm? Absolutely. Caffeine is a stimulant that reverses the effects of adenosine, which works on behalf of the circadian cycle on a molecular level. What this means is that you may not be able to fall asleep.

Does Stress Disrupt the Circadian Rhythm? 

Yes, stress disrupts circadian rhythm. Your risk of metabolic disorders, including diabetes and obesity, increases when high-stress levels interrupt the circadian cycle. What causes weight gain is the negative impact that stressors have on your metabolism. Adding lack of sleep into the mix creates a problem.

The relation between circadian rhythm and the immune system in the morning is complex. Simply put, the body must refresh to be able to fight against infection. Your immune system peaks in the early mornings. Interruptions in the sleep cycle can increase inflammation and lead to illness. 

Does Insomnia Disrupt the Circadian Rhythm? 

Yes, insomnia disrupts circadian rhythm. Insomnia is another condition where you cannot regulate circadian rhythm. Inability to fall asleep can delay the cycle and throw everything off. 

Who Is the Inventor of the Circadian Rhythm?

Which psychologist found the circadian rhythm? In 1729, French astronomer Jean Jacques d’Ortous de Mairan used a mimosa plant to observe how light and dark affect behavior. During that experiment, the plant would open its leaves in the morning and close them at night.     

Over the next 200 years, other discoveries determined what caused the external sources to change behavior. Scientists Jeffery C. Hall and Michael Rosbash of Brandeis University and Michael Young of Rockefeller University used drosophila circadian rhythm tracking to demonstrate what type of actions the fruit fly makes throughout the sleep cycle.                

How Can Circadian Rhythm Affect the Lifespan of a Human Being?

Circadian rhythms change as people age. The average circadian rhythm for teens is late, about age 19, and demonstrates that students who stay up late tend to perform lower than those who choose an earlier bedtime. 

How Can Circadian Rhythm Affect the Lifespan of a Human Being
How Can Circadian Rhythm Affect the Lifespan of a Human Being?

As you get older, your circadian cycle shortens. Older adults tend to be more productive first thing in the morning and go to bed earlier at night than younger adults and teens. According to the Sleep Foundation, circadian rhythm changes shorten by about thirty minutes every ten years once you reach middle age. 

Visually-challenged children typically have difficulty sleeping because they cannot distinguish between light and dark. A way to treat a sleep disorder is to change the type of lights a person uses. 

However, light therapy for circadian rhythm blind children may not work if there is no light perception. In this case, setting a bedtime and wake-up schedule would be beneficial. Cold showers or exercise early in the morning may increase alertness. Another favorable option is extended-release melatonin. 

Can Circadian Rhythms Affect Exercise Efficiency?

Exercise efficiency is the work rate over the energy you use to reach your target. Circadian rhythm has a positive impact on exercise efficiency via timing. 

Studies showed that the time of day makes a difference in the quality of exercise. According to researchers, humans perform better in the evening due to less oxygen consumption. 

Exercise improves the function of mitochondrial content in the muscles. Physical activity increases the oxygen flow to the tissues, including muscles, where the mitochondria reside.  When the mitochondria are healthy, the body can use oxygen more efficiently. 

Circadian rhythm can have mental effects on exercise. If your sleep cycle is off, your mood may suffer. You may not have an effective workout. 

Can athletes use circadian rhythm to improve their talents? Yes, they can. The time and place of the game contribute to performance. 

Experts recommend athletes should adjust to the time change. If the game is in a different time zone, players should plan to arrive two to three days early. Time can matter as well. Older players perform better in the morning, whereas younger athletes prefer later times.

How Does Exercise Influence the Circadian Rhythm?

Exercise exerts a profound influence on the circadian rhythm, serving as a powerful zeitgeber or time cue that helps synchronize the body’s internal clock to the environment. Engaging in regular physical activity, especially in the morning, can advance the phase of the circadian rhythm, promoting earlier onset of sleep and wake times. This adjustment can enhance alertness in the morning and improve sleep quality at night.

However, the timing of exercise is crucial; while morning exercise has a positive effect on the circadian rhythm, exercising close to bedtime can have the opposite effect, potentially delaying sleep onset. The mechanism behind this involves exercise-induced increases in body temperature and the stimulation of alertness-promoting hormones, which can disrupt sleep if occurring too late in the day.

What Foods Reset or Disrupt Your Circadian Rhythm?

Certain foods have the potential to reset or disrupt the circadian rhythm due to their impact on sleep-wake cycles and hormone regulation. Foods rich in tryptophan, such as turkey, chicken, and nuts, can support the production of melatonin, the sleep hormone, thereby aligning the circadian rhythm with nighttime sleep.

Conversely, caffeine and high-sugar foods can disrupt the circadian rhythm by interfering with sleep quality and timing. Caffeine, found in coffee, tea, and chocolate, blocks adenosine receptors, preventing the onset of sleepiness and potentially delaying sleep. Consuming these foods late in the day can therefore lead to a misalignment between the internal circadian clock and external light-dark cycles, impacting overall circadian rhythm regulation.

Can Specific Supplements Help Regulate Your Circadian Clock?

Yes, supplements can help regulate the circadian clock by supporting the natural production of sleep-related hormones and adjusting the body’s internal timing mechanisms.

  • Melatonin supplements: Melatonin supplements are commonly used to adjust sleep cycles, particularly in cases of jet lag or shift work, by mimicking the body’s natural sleep signals.
  • Magnesium supplements: Magnesium supplements are known for their role in supporting sleep quality, and can also assist in circadian rhythm regulation by promoting relaxation and reducing sleep latency.
  • Omega 3: Omega-3 fatty acids, which are found in fish oil supplements, may influence sleep patterns and circadian alignment through their effects on serotonin production.

However, while these supplements can be beneficial, they should be used judiciously and ideally under the guidance of a healthcare professional to ensure they are appropriate for individual health needs and circadian rhythm objectives.

How Do Behavioral Changes Impact Circadian Rhythm Regulation?

Behavioral changes can have a significant impact on circadian rhythm regulation by reinforcing natural sleep-wake cycles and promoting alignment with external light-dark cycles. Establishing a consistent sleep schedule by going to bed and waking up at the same time every day strengthens the circadian signal, enhancing sleep quality and daytime alertness.

Exposure to natural light, especially in the morning, reinforces the alignment of the internal clock with the external environment, boosting mood and energy levels. Conversely, minimizing exposure to blue light from screens before bedtime can prevent the disruption of melatonin production, facilitating a smoother transition to sleep. Implementing these behavioral adjustments can greatly improve the regulation of circadian rhythms, promoting overall health and well-being.

Athletic Insight

Athletic Insight Research


The Athletic Insight Research team consists of a dedicated team of researchers, Doctors, Registered Dieticians, nationally certified nutritionists and personal trainers. Our team members hold prestigious accolades within their discipline(s) of expertise, as well as nationally recognized certifications. These include; National Academy of Sports Medicine Certified Personal Trainer (NASM-CPT), American College of Sports Medicine (ACSM), National Strength and Conditioning Association (NSCA-CPT), National Academy of Sports Medicine Certified Nutrition Coach (NASM-CNC), International Sports Sciences Association Nutritionist Certification.