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. Such an important aspect will be introduced and reviewed in this post.. Get your protein shake, stay comfortable and enjoy this 5min read.


As we usually do (and you should know it by now), here we like going through the definition before getting into more deep content. So now the question is: what are Circadian rhythms?

Following the definition given by the National Cancer Institute (NCI), circadian rythms are the natural cycle of physical, mental, and behavior changes that the body goes through in a 24-hour cycle.

Following this definition we can get an idea on how important they can be for our organism, and disruption of these may lead to adverse health effects. But with a brief knowledge of how the circadian clock works you can easily manage to optimize them and avoid the adverse effects.


Almost all metabolic reactions are constrained to this 24h cycle. 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.

Our biological 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.

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


Circadian rhythms are regulated by many external factors, which are captured by the ganglion cells in the retina and transmitted to the supraquiasmatic nucleus (in the brain) and central nervous system (CNS) to secrete hormones and neurotransmitters (source).

As we can see in the image, we are highly regulated at any moment of our lifes. 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 rythms.

But how is all this controlled? Our organism is highly controlled and regulated, and this is not an exception. Here we'll see how we can control and regulate the Circadian 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.


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.

In fact, the most known action of the light influence in our biological clock is sleep. When light is present, our body interprets that as time for being active, stops secreting melatonin and starts secreting serotonin. Inversely, 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 our biological clock.


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 aminoacid 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. When intense light is available, production of melatonin decreases (daylight). On the other hand, absence of intense light leads to an increase in melatonin production (study).

The problem we find in the actual society is the abuse of intense and artifical light, mainly coming from electronic devices. In this study made with college students, it was found that those using computers late at night showed a significant decrease in melatonin levels (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 exercise and Circadian Rhythms are highly correlated. Not only photic stimuli such as light play a role regulating, but other such as exercise can help regulating the circadian clock. These non-photic stimuli are captured by peripheral clocks situated in the majority of our tissues, and the messages will be sent to the central clock to regulate the circadian clock (source).

Induced exercise during daylight can phase-shift your melatonin production to regulate it based on your circadian rythms (study). In this study, it was seen how physical exercise helped entraining disregulated circadian rhythms. Therefore, if you are a night-worker or simply have your circadian clock disregualted, physical exercise is a great tool to go back on track and 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.


Chrononutrition and meal timing has been gaining importance lately, showing that it might affect overall health and other parameters of interest such as insulin sensitivity. Although this is beyond the scoop of this post, there is some emerging evidence that meal timing can affect the circadian clock as well.

Human metabolism, nutrition and circadian rhythms are interlinked. Molecules related with metabolic processes and food intake such as adiponectin are regulated by circadian rhythmicity (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, enhancing your fat loss and other cardiovascular and glycemic parameters such as insulin sensitivity.

Sadly we don't have enough evidence to set a final protocol to potentiate these benefits. The major evidence is shown in rodents. Although their metabolism is fairly similar to ours, we cannot extrapolate results without having sufficient studies and evidence in humans.


We find a high subject variability when it comes to Circadian Clocks. Everyone shows a different Circadian Clock, and -as everything on human health- it should be put into the context of each person.

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

Normal distribution showing different chronotypes and the frequency of these (source)

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. Interestingly, in this study, after surveying nearly 500 participants, they show that almost everyone showed sleep deprivation during work-days (study). Here we can prove how or 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.


Dysruption of circadian rythms

Disrupting Circadian Rhythms is simply one of the worst things you can do for your health and metabolism. It's 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 new life, mainly because it's probably chaotic and doesn't show any regular pattern.

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 disregulation with the environment and circadian rhythmicity. I hope after you learn a bit about how important they are you take care of yourself a bit more.


Although I won't take you deep into how it can affect our health (you'll have a whole post abou it soon ), I want to give you a quick list with the evidence showing the main adverse effects of disregulating the Circadian Rhythms.

+Favours 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.

The main problem resides in the dysruption taking place between the actual society and rest of the enviroment. We are generally pointing towards dysrupted Circadian Rhythms, and that can lead to severe health effects.

In future posts we'll go deeply into how it can affect our health and what to do to avoid the 'dark-side'.

If you have any doubts, please leave your comment below in the comment section and I'll answer as soon as possible. See you next week with another exciting topic to talk about.

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

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