Circadian Rhythm System – How Does It Work?

Wondering how does your internal body clock work? Let’s take a closer look at how this circadian system works, the different factors that can impact it, and what you can do to boost it and use your biology to improve other aspects of your life.

Everything on Earth works in cycles. From a rather uniform day and night cycle to changing seasons; everything appears to be repeating. All living things had to adjust to the changing environment, and they seem to have internal clocks that can work on their own but are also synchronizing based on the external cues. The Sun is the most crucial environmental stimulus, and most organisms perform their activities depending on the day and night cycle.

Circadian rhythms are described as repeating patterns that occur daily. The term is coined from the Latin “circa” meaning “around,” and “diem” meaning “day.” Most living things have an internal clock that is controlling every activity based on these rhythms. Only a small portion of organisms that live in coastal regions are synced to lunar days and don’t follow this pattern. Instead, they depend on the Moon and its gravitational impact on tides. A lunar day is a time it takes the Moon to finish the rotation around its axis compared to the Sun. It is slightly longer than the day, and its duration is 24 hours and 50 minutes.

Let’s take a closer look at how this circadian system works, the different factors that can impact it, and what you can do to boost it and use your biology to improve other aspects of your life. 

How Does Our Internal Clock Work?

The master clock is located in the suprachiasmatic nucleus (SCN), which is a part of the hypothalamus, a region that plays a role in regulating many vital functions like body temperature, appetite, thirst, sleep cycle, sex drive, emotions, blood pressure, heart rate, and many more. If we were to describe what hypothalamus does, we would say that it is in charge of keeping the homeostasis, the principle on which all organisms function. Homeostasis represents the dynamics of the body that uses various feedback mechanisms to keep everything in balance. It doesn’t mean that the state of your body is always the same, but that it is within specific parameters.

The SCN is the size of a pea, and it contains around 20,000 small neurons that send signals to other parts of the brain to regulate sleep/wake cycle, hormone production, body temperature, and other functions. Each neuron exhibits a near 24-hour activity rhythm, which suggests that the clock mechanism works even on a cellular level. When these brain cells are individually grown in a Petri dish, they each follow a 24-hour rhythm, but when they are incorporated in the SCN, they all synchronize together. In the experiments where researchers would remove the SCN in mice, the animals that are otherwise active at night and sleep during the day, show little to no preference in these activities. Their activity didn’t seem to follow a pattern and was random through both day and night. 

This biological clock needs to check its accuracy each day and to do that it uses external stimuli, mostly the sunlight. There are specific photoreceptive ganglion cells in the retina of the eye, which are entirely different than rods and cones that are in charge of generating pictures of the world around us. These cells contain a light-sensitive pigment called melanopsin, that is most sensitive to blue light. Exposure to daylight stimulates a pathway from these ganglion cells, and send a signal about the time of the day so that our biological clock can adjust. Interestingly, most blind people can sense these stimuli, since these photoreceptors can recognize daylight even through closed eyelids. The signals are sent via the optic nerve to the SCN, and it then regulates itself accordingly.

Our internal clocks don’t need light to function, and the circadian rhythms exist even if the person is cut off from the daylight completely. However, the length of day and night varies during the year, and with different geographic locations, so light in addition to other external cues like temperature and meals, serve to synchronize with your environment and prevent small timing errors. Without this feedback, the circadian system could become unbalanced and cause some problems. For instance, shift workers can never fully adapt to their irregular sleep patterns and face many health problems simply because artificial light is not as efficient in resetting the circadian clock in these people. It has been shown that increasing natural daytime lighting in elderly care homes can significantly improve mood disorders, sleep patterns, and can reduce cognitive decline. That is why scientists suggest that everybody should spend some time outside in natural light, as it appears to have many benefits.

Other secondary biological clocks are located throughout the entire body, and they can be found in the heart, liver, kidneys, pancreas, intestines, lungs, lymphocytes, and skin. They are affected by other cues other than light, such as meal times and environmental temperature. The SCN receives feedback from these secondary clocks and uses the information for further synchronization and coordination. Chronobiology is a relatively new discipline that found practical use in the medical field. It turns out that the effectiveness of treatment can significantly increase if it’s applied at the right time, depending on the involved organ.

Circadian Rhythms in Animals

Other living beings on Earth experience the very same natural rhythms that we do. From single-celled bacteria, through plants, to other animals, every organism seems to have an internal clock that regulates its activity. That became evident with the first simple experiments that took place in the 18th century. Curious researchers decided to put plants in the dark to see what would happen, and it turned out that plants tried to adjust to new conditions, but also kept some of the previous activities that otherwise wouldn’t occur during the night. 

The modern-day research of circadian rhythms is mostly done on fruit flies (Drosophila). We have learned a lot about how different genes, biochemistry, and environment all impact the internal clocks. 

In birds, mammals, and reptiles, the central biological clock is located in the hypothalamus, while other organisms don’t have this brain part, so it is located somewhere else, or the rhythms are regulated on the cell level in bacteria. The light is sensed with eyes, or pineal gland in some animals. It is located at the top of the head, and it can also perceive light stimulus and send the information to other parts of the brain. 

Animals need these signals to adjust their activities. For instance, rodents are active during the night, and they use their strong sense of smell to forage for food. Many predators like owls and felines are also active during the night, and they hunt rodents and other animals that are active at that time. Animals that mostly rely on their vision use daytime for their activities. Herbivores eat throughout the whole day and then rest during the night, as they can’t see as clearly, and they would be easier prey for predators. Interestingly, squirrels are most active at dusk and dawn, and they avoid overexposing both during the day and night.

A presence of light is essential for circadian rhythms, but it also signals changes in seasons, and most animals combine it with other environmental cues such as temperature to prepare for mating, hibernation, or migration. That ensures they survive the unfavorable period, and leave the offspring when the conditions are most suitable for their survival.

It is interesting how different animals adapted to conditions where the light is not an adequate stimulus. For instance, deep sea and cave-dwelling fish don’t experience sunlight in their environment, and their internal clocks seem to be operating on its own without being based on the presence of light. Also, reindeer and other animals that live in polar regions with long daylight periods during summer, and exceptionally long nights during winter, seem to be independent of this day/night cycle. 

Genetic Background 

Many different genes that play a role in maintaining circadian rhythms have been identified in recent years. It is no surprise that they are mainly active within the cells of the SCN, but they are also found in other tissues of our bodies. Scientists estimate that around 15% of our genes operate on a 24-hour cycle, and CLOCK, CRY, TIM, PER, and BMAL have been identified to play an important role in the process of sleep. Although scientists are still not sure about all different mechanisms through which they regulate sleep, research in this field is very active. It is known that mutations in these genes are closely linked to several different sleep disorders.

Research of different genetic markers in all organisms discovered something interesting. Even though mammalian clock genes are similar to those of most living things, there is one group that stands out. There is still no connection between genes that regulate circadian rhythms in cyanobacteria, which leads to the conclusion that these systems have evolved independently at least two times. However, there is more research needed to shed light on what is happening there, so we look forward to discovering more about it.

Unexpected Consequences of Circadian Rhythms

Many processes in our bodies occur rhythmically. Our internal clock regulates when to go to sleep and wake up, controls alertness, food intake, temperature regulation, organ functioning, and hormone production. Some other processes are linked to circadian rhythms, even though it might not seem like that at first glance:

  • Giving birth usually occurs at night, because the evolution has timed hormones to trigger labor at this time. There is a simple explanation behind this, a mother and her newborn are probably less vulnerable in the dark as they are not as visible to predators, which increases their chances of survival. Even though modern day humans don’t have this problem, it appears that deliveries after midnight are still more frequent.
  • Blood pressure spikes in the morning, which generally has the effect of waking us up and promoting alertness, but it also increases the chances of heart attack and stroke. 
  • Allergies are often worse in the morning. As much as 70% of allergy sufferers report more symptoms such as sneezing and runny nose in the mornings.
  • Asthma attacks are also more prevalent at dawn. One of the possible explanations is that the cortisol levels are very low at that point, which is a steroid hormone that plays a significant role in battling inflammation.

These and many other discoveries lead to the development of chronotherapy, which aims to use natural rhythms to predict the best time to take medications for maximal effectiveness.

Circadian Rhythm Disorders

Several different conditions limit the body’s ability to get sufficient rest. People with circadian rhythm disorders suffer from the lack of sleep, too much sleep, or simply not resting enough at the right time. Their internal clocks are off for some reason, and that can lead to a series of consequences. Here are the most common circadian rhythm disorders:

  • Delayed sleep phase syndrome (DSPS) is relatively common, and some estimations show that around 15% of teenagers in the United States suffer from it. People with it usually have 3 to 6 hours delay of sleep onset time. Naturally, they also wake up later. They are getting sufficient sleep, it’s just that their internal clocks are not synchronized with the environment, and they follow their schedule. This behavior is usually frowned upon, and people with DSPS are often labeled as lazy and unmotivated. Because their rhythm is different than what is socially acceptable, they often fail at fitting in their responsibilities. By sacrificing sleep, they can risk the development of insomnia, and people with DSPS often suffer from depression and anxiety disorders.
  • Advanced sleep phase disorder (ASPD) is pretty much the opposite of DSPS. People affected by this condition usually experience sleep onset between 7 pm and 9 pm, and they wake up around 3 am. Since this schedule fits in what is socially acceptable, these individuals usually don’t experience as much backlash as people with DSPS, but social alienation can lead to depression. There is a strong genetic link behind this disorder, and people suffering from it have a 50% chance of transferring it to their offspring. 
  • Non-24-hour sleep-wake disorder affects the synchronization of the body clock with the environment. Affected people usually have slightly longer than the usual 24-hour day/night cycle, with the gradual expansion. They have trouble falling asleep most of the time but can get a good quality sleep a few times a month when their clock aligns with the photoperiod, and they can feel temporary relief.
  • Irregular sleep-wake rhythm is characterized by the complete absence of regular sleep patterns. There is no prolonged sleep period during the night, and sleeping times seem sporadic and random. This condition is extremely rare and is usually in correlation with certain neurological disorders like Alzheimer’s, mental retardation, dementia, and brain damage.
  • Shift work sleep disorder affects people working at night and those who are always rotating shifts and don’t have a steady schedule because of it. These people are often sleep deprived, have a higher chance of developing insomnia, and experience excessive sleepiness during waking hours. They are also at a higher risk of having a driving or work-related accident because of the drowsiness and decreased concentration.

The disruptions in circadian rhythms can be a consequence of traveling through several time zones. Jet lag can affect everybody, and people usually have trouble adjusting to a new schedule for several days. Other factors include pregnancy, medication, changes in daily routine, mental health problems, menopause, medical problems like Alzheimer’s and Parkinson’s disease.

Disruption of Sleep/Wake Cycle and Mental Health

Everybody who has missed a night or two of sleep knows that it affects the ability to function the next day. Lack of sleep affects your mood, it makes it harder to concentrate, memorize things, and decreases your work and academic performance. Long term effects go far beyond feeling moody and distracted.

The disruptions of circadian rhythms can affect our physical and mental health. Sleep deprivation is linked to the increased risk of developing anxiety, bipolar disorder, and depression. Increased activity during the resting hours and inactivity during the day have the strongest connection with the higher risk of mood disorders, slower reaction times, and worse subjective feelings of well-being.

Unfortunately, it looks like more and more people are experiencing both mental health problems and sleep disruptions. The faster way of life has got us all chasing around, and sacrificing precious rest to do as many activities as we can. Here are some things you can do to sync your internal clock with the environment and enjoy better sleep, health, and the subjective feeling of well-being:

  • Try to maintain a stable sleep schedule. Going to bed and waking up at the same time can help your brain realize when to rest and when to be active. 
  • Create a relaxing nighttime routine that will help you unwind and fall asleep faster. You can try reading, meditating, or listening to music. Ban the electronics from your bedroom, and keep it dark, quiet, and cold, to promote a distraction-free environment.
  • Control your light exposure. Walking outside the first thing in the morning will get you some sunlight, and it should help your clock restart. Consider spending some time outdoors in the natural light each day for the best results. You can try getting smart light bulbs at your house since you can adjust the light they emit. That way, you can dim them and set a warmer shade in the evening to let your brain know that it is time to go to sleep. That should boost the production of melatonin. Avoid screens at least a couple of hours before bedtime as they emit blue light that can confuse your brain into thinking that it is the morning, which is the time to be active. 
  • Try to exercise daily. Only 20 minutes of moderate exercise can do wonders for your sleep and overall health, so try to incorporate it into your everyday routine. Don’t do it too close to bedtime, as it can make it harder to fall asleep since it takes some time for your body to cool down.
  • Enjoy a balanced diet with whole grains, lean protein, and plenty of fruits and vegetables. It is best to create a schedule for your meals as well; consume them in a 10-12 hour window and then leave 12-14 hours for your body to rest and repair. Avoid nicotine, alcohol, and caffeine in the second part of the day since these stimulants can disrupt your sleep.


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