circadian rhythms

Introduction and Regulation of Circadian Rhythms

Why do our days have 24h and all our actions and life are constrained to that range? The answer is Circadian Rhythms. But what are Circadian Rythms, and why are they so important?

They are the main reason of our days having 24 hours, our needs to sleep and many more actions we do in our everyday life.

Get your protein shake, stay comfortable and enjoy this 5min read.

What are the Circadian Rhythms?

Circadian Rhythms are the natural cycle of physical, mental, and behavior changes that the body goes through in a 24-hour cycle (source)

The circadian rhythms are an important player for our health, and disruption of these may lead to adverse health effects.

With a brief knowledge of how the circadian clock works you can easily manage to optimize them and avoid the adverse effects.

Introduction to Circadian Rhythms

Almost all metabolic reactions are constrained to the Circadian Rhythms. In fact, they show different activity depending on the moment of the day.

This way we can synchronize with the environment and use metabolic energy more efficiently. All mammals show a circadian clock, which states the importance of these for the survival and evolution of a species.

Circadian Rhythms

The Circadian Clock is built by a central clock, situated in the supraquiasmatic nucleus, and other peripheral clocks found in peripheral tissues of our organisms.

The supraquiasmatic nucleus is in charge of recieving all the stimuli from the peripheral clocks to regulate and synchronize the metabolism.

How are Circadian Rhythms Stimulated?

We have two main types of stimulus to synchronize the Circadian clock: photic and non-photic.

Photic stimulus (light) is directly sent to the central clock. The non-photic are more likely to be captured by peripheral tissues, and from there to the central clock.

Although photic stimuli are known to be the main regulator of Circadian Rhythms, non-photic such as physical activity or food intake seem to have a major role of regulation and phase-shifting the cycle as well. (source)

How the Circadian Rhythms Work

The regulatory factors are captured by the ganglion cells in the retina and transmitted to the supraquiasmatic nucleus (in the brain) and central nervous system (CNS). This will secrete hormones and neurotransmitters (source).

We are highly regulated at any moment of our day. The variables changing throughout the 24h are, among others, melatonin and serotonin secretion, body temperature, blood pressure, intestinal activity, coordination and alertness.

All these responses are called zeitgebers, playing an important role synchronizing our biological rhythms.


  • Circadian Rhythms synchronize our actions within a 24h cycle

  • It's built by a central clock and other peripheral clocks interacting between them.

  • Circadian rhythms are regulated by zeitgebers

  • The supraquiasmatic nucleus is in charge of syncronizing and sending all the orders to the rest of our tissues.

Light/Dark Cycles

As main regulator of the Circadian Clock we have the light/dark cycle. This is the perfect example on how we -humans- are connected to natural phenomenons such as solar light.

Light, considered the main zeitgeber, is detected by the glangion cells in the ocular retina. The signals produced by this structure is transmitted to the suprachiasmatic nucleus (where the 'clock' is found) to synchronize de cycle to approximately 24h.

From the suprachiasmatic nucleus, the signals are transported to different organs to synergically act and regulate our organism.

When light is present, our body interprets that as time for being active, stops secreting melatonin and starts secreting serotonin.

When light goes off and night is coming, melatonin production increases and serotonin production decreases (study).

This is the main mechanism by which light and darkness can affect the circadian rhythms.


Melatonin is the major hormone regulating our sleep. The production and release of this hormone takes place in the pineal gland, and it's synthesized mainly from the amino acid tryptophan and the hormone serotonin.

This hormone plays an important role in the sleep-wake cycle. It's highly regulated by the amount of light being captured by our eyes.

melatonin and circadian rhythms

When intense light is available, production of melatonin decreases (daylight). Absence of intense light will lead to an increase in melatonin production (study).

If melatonin production is supressed or lower than normal, it could lead to disruption of Circadian Rhythms. Being exposed to intense lights before going to bed can confuse our body, and it may think that it's still daylight.

Physical Activity

Physical exercise and Circadian Rhythms are highly correlated.

Induced exercise during daylight can phase-shift your melatonin production to regulate it based on your circadian rhythms (study).

In this study, it was seen how physical exercise helped entraining dysregulated circadian rhythms.

If you are a night-worker or simply have your circadian clock dysregulated, physical exercise can help you avoid the consequences of a disruption on the Circadian Rhythms.

Physical exercise induces a phase-shift on serum melatonin levels

In this figure (taken from the study mentioned above), we can see how the subjects performing physical activity (black dots) showed a phase-shift on melatonin levels, while the control group (white dots) shows a similar production overtime.

Food Intake

Human metabolism, nutrition and circadian rhythms are interlinked. Molecules related with metabolic processes and food intake such as adiponectin are regulated by the circadian rhythms (study).

Adiponectin is a hormone correlated with energy metabolism, affecting blood pressure, glycemic and lipidic metabolism and fat loss (study)(study).

Time-restricted intake protocols have been studied and correlated with a regulation of disrupted circadian rhythms.

Food intake and energy metabolism could favour the regulation of the circadian clock.

Different Chronotypes

There´s a huge subject variability when it comes to Circadian Clocks. Everyone shows a different Circadian Clock, and that should be put into the context of each person.

xlvii. Distribution of early and late types | DSPS, a sleep disorder

Humans have different chronotypes. Although it shows a normal distribution, there are some extreme early-type and extreme late-type persons.

For these subjects, general recommendations cannot be totally applied, and adherence and personal feeling should go above everything.

Human chronotypes are highly influenced by certain variables. Between these, we have lifestyle, location, culture, age, gender, physical activity, nutrition,...

Chronotypes also have a genetic component, although is highly overpassed by the environment.

In this study, after surveying nearly 500 participants, they show that almost everyone showed sleep deprivation during work-days (study).

Our lifestyle and routine can overpass our genetics to adapt to the environment.


  • The main regulator of Circadian Rhythms is light.

  • Melatonin production is supressed when abuse of light late at night, compromising sleep quality and duration.

  • Food intake regulates circadian rhythmicity. Meal timing and restricted-feeding can alter glycemic and lipidic parameters

  • Physical activity is a great tool to regulate circadian rhythms if disrupted.

  • Not everyone shows the same chronotype. However, our actions and lifestyle overpass genetics.

Disruption of Circadian Rhythms

Dysruption of circadian rythms

Disrupting Circadian Rhythms is  one of the worst things you can do for your health and metabolism.

Circadian Rhythms are the main mechanism by which we are synchronized and built within an environment.

If we disrupt this interconnection we will not get the benefits from environmental factor and our body won't be able to adapt to the environment we live in.

Disruption of Circadian Rhythms as a Pandemic

Circadian Rhythmicity is a real pandemic and we haven't realized yet. Low quality sleep, abuse of electronic devices, night-shifts, chronic stress, sedentarism,...

Our actual society is pointing towards a dysregulation with the environment and circadian rhythmicity.

Main Adverse Effects of Disrupting Circadian Rhythms

Circadian Rhythms plays a big role in our health. Disrupted Circadian Rhythms can:

+Favour weight gain and slows fat loss (study)

+ Higher prevalence of cardiovascular events (study)

+ Irregular sleep patterns and worse quality of sleep (study)(study)

+ Higher risk of sleep disorders (source)

+ Association with mental disorders such as autism and depression (study)(study)(study)

+ Higher prevalence of obesity and metabolic diseases (study)(study)(study)

Adverse Effects of Dysregulated Circadian Rythms


In this post we have seen how important Circadian Rhythms are for our survival and adaptation to the environment.

As many other systems, Circadian Rhythms are highly regulated by factor such as light, physical activity or food intake. Additionally, they are highly influenced by the chronotype of each subject.

We are generally pointing towards disrupted Circadian Rhythms, and that can lead to severe health effects.

1 thought on “Introduction and Regulation of Circadian Rhythms”

  1. Pingback: Sleep deprivation: does it affect your trainings? - TrainingbyScience

Leave a Comment

Your email address will not be published. Required fields are marked *