What are circadian rhythms?
Circadian rhythms are biological cycles of approximately 24 hours that form part of our internal clock. They regulate many biochemical, physiological and behavioural functions and processes in the body, such as the sleep–wake cycle. Examples of circadian rhythms include body temperature, the secretion of certain hormones such as cortisol or melatonin, heart rate, blood pressure, muscle strength, and the rhythmic activity of the digestive system.
Mechanisms that control our rhythmicity
All circadian rhythms are governed by a small region of the brain known as the suprachiasmatic nuclei (SCN). These act as a central clock which, influenced by the amount of sunlight perceived through the retina and other signals from the environment, plays the role of an orchestra conductor. This “central pacemaker” coordinates and synchronises other biological clocks or peripheral oscillators throughout the body. Some of these are located in the heart, pancreas, adipose tissue, digestive system, kidneys and liver.
Although the suprachiasmatic nuclei and other biological clocks function as oscillators themselves, with a circadian cycle lasting approximately 24 hours in humans, their rhythmicity is influenced by so-called zeitgebers, which are external factors that “set” these internal clocks.
Sunlight, for example, influences the secretion of a hormone called melatonin. During daylight hours, increased light exposure causes the pineal gland to stop secreting this hormone. As night begins and light levels decrease, the opposite occurs and melatonin levels rise significantly, producing the sudden need to sleep. Throughout the night, melatonin levels gradually fall, allowing us to wake up in the morning.
The light-dark cycle is one of the most important periodic inputs of information for the proper functioning of our central clock. Any habits that disrupt this pattern, such as increased night-time activity, reduced sleep duration, or exposure to artificial light sources like electronic device screens, can impair its regulation.
In addition to sunlight, there are other factors capable of synchronising these internal clocks, such as meal timing, which is the most important synchroniser for peripheral oscillators. Other synchronisers include the activity-rest cycle and social interactions.
What is chronodisruption and which factors cause it?
Changes in society, such as the introduction of artificial light, a more sedentary lifestyle and the need to adapt to work schedules, have led to alterations in the factors that synchronise and regulate our internal clock.
Let’s look at an example. Imagine you are a young woman in the early 20th century who works from sunrise to sunset (literally) in the fields. You would likely wake up at dawn, eat something and head out to work early. At midday, you would stop to eat with your family and take a short nap to be rested for the rest of the day. After an intense afternoon, you would return home, have dinner as it got dark, and share the day’s stories with your neighbours. After chatting, already tired, you would go to bed. And all of this would happen at roughly the same time every day.
Now let’s travel to the 21st century. You are a security guard working shifts at a city hospital. Last week you were on the morning shift, and no one could get you out of bed at 6 a.m. This week you are on the afternoon shift and you take advantage of it by sleeping until midday. Because there is a lot of work and your schedule varies greatly, you eat at different times each day, often at very irregular hours. When you get home in the evening (if you are not on call), you usually grab something to eat and watch that TV show you love until late. During the day you are mostly sitting down and only exercise occasionally.
Between these two stories there are key differences: the presence or absence of stable routines, and whether light exposure patterns are disrupted or not. The stability of synchronisers strengthens, stabilises and optimises the functioning of the internal clock by keeping it properly aligned, whereas a lack of stability disrupts its activity and leads to misalignment of the circadian system. This desynchronisation is known as chronodisruption, and its onset has consequences for our health.
Chronodisruption increases the risk of developing certain types of cancer (especially colon and breast cancer), obesity, metabolic syndrome, affective disorders (depression and anxiety), cardiovascular disease and sleep disorders. Disruption of circadian rhythms also weakens the immune system, making us more prone to infections and the development of autoimmune diseases. In addition, it can contribute to cognitive alterations such as reduced concentration and memory problems.
Although it is true that we are all susceptible to chronodisruption in our daily lives, there are genetic factors that can make some people more sensitive to its effects.
How to prevent chronodisruption
The best way to keep our circadian rhythms properly aligned is to ensure that our internal clocks and oscillators receive the appropriate cues to stay in sync.
To do this, the most important steps are to expose yourself to natural daylight during the day and to avoid very bright lights and the use of electronic devices at night, especially before going to bed. These devices emit types of light, such as blue or white light, that inhibit melatonin secretion, making it harder to fall asleep. Warmer or more amber-toned lighting is the best option for evening and night-time illumination, as it more closely resembles the natural light at the end of the day.
In addition, it is advisable to maintain regular sleep and meal schedules. This also applies to weekends and holidays, when we tend to follow different routines from the rest of the week. Doing so helps prevent chronodisruption that could affect sleep quality or metabolism.
Another key synchroniser is the activity-rest cycle. Engaging in regular, moderate physical exercise provides a strong cue for our internal clocks, although it is best to avoid exercising too late in the day, as it may be overly stimulating and interfere with sleep.Finally, do not neglect social interactions, as in addition to improving emotional wellbeing, they also act as an effective chronopotentiator.
