Circadian rhythm: what it is and how it works

Why are we hungry around lunchtime and sleepy at dusk?

The answer lies in our circadian rhythm – popularly known as the biological clock -, which regulates the body’s daily activities in a period of (approximately) 24 hours.

This internal mechanism is present in all living things – and it keeps our bodies running on a schedule, day and night, wherever you are.

“Essentially, it’s the internal biological representation of a day,” explains Russell Foster, professor of circadian neuroscience at the University of Oxford in the UK, to BBC Radio 4.

“It provides a temporal framework for our biology to function effectively. We have to deliver the right substance, in the right concentrations, to the right organs, at the right time of day. And it is our biological clock, our circadian system, that allows us to organize this temporally. , this temporal structure.”

It therefore dictates the rhythm of virtually every oscillation that happens in our body – in body temperature, blood pressure, hormone levels, urine production, heart rate, and so on.

And so it guides us in relation to the best time of day for certain activities, such as sleeping, eating and thinking.

Where is this watch?

Our circadian rhythm is governed by a master clock located in the brain – more specifically, in the SCN (suprachiasmatic nucleus), within the hypothalamus.

And each cell in the body, in turn, has its own clock, the so-called peripheral clocks, which follow the rhythm of the master clock.

“The master clock acts a bit like the conductor of an orchestra, producing a regular temporal beat, from which all the billions of cells in the body, in all the organ systems of the body, coordinate this rhythmic activity and align it to the 24-hour rotation of the Earth”, explains the expert.

more than 24 hours

But our internal clock doesn’t exactly have 24 hours. According to Foster, the hands rotate a little faster or slower depending on the individual.

To be more precise, a study from Harvard University in the USA showed that the circadian rhythm has an average of 24 hours and 11 minutes – based on the daily monitoring of the participants’ hormones and body temperature.

Therefore, it is necessary to adjust the hands of our master clock daily, so that they are in sync with the 24 hours of the day.

And it’s our daily exposure to sunlight – or rather the natural light (day)/dark (night) cycle – that keeps us in sync with the outside world, setting our internal clock.

Basically, a group of photosensitive cells present in the retina of the eye detect ambient light, providing information via the optic nerve to the brain about the length of day and night – which helps ensure that our internal clock is kept in sync with the day. 24-hour astronomical

Without this mechanism, the extra 11 minutes of our circadian rhythm would accumulate and you would wake up and go to sleep a little later each day.

Because it’s important

The adaptive advantage of having an internal clock is that it allows our bodies to anticipate certain predictable events – such as the sleep-wake cycle and meals, for example – and prepare for them.

When night falls, our body prepares for rest, and starts producing melatonin, the sleep hormone. At dawn, however, it releases cortisol to wake us up.

The schedule of hunger hormones – leptin (responsible for satiety) and ghrelin (for opening up the appetite) – follows the same principle, or rather, the same clock.

According to Foster, recent studies show that leptin release peaks around 2:00 a.m., and the lowest point at midday.

“Typically, high levels of leptin at night suppress your appetite so hunger doesn’t interfere with your sleep.”

“Meanwhile, ghrelin is secreted by the stomach during the day and increases in anticipation of meals,” the expert wrote in an article for the British newspaper Daily Mail.

He reminds us that not respecting this natural rhythm can have serious consequences for our body.

Research shows that living out of sync can make us more vulnerable to health issues like infections, cancer, obesity, type 2 diabetes, heart disease and mental health disorders like depression.

It is a real challenge for those who work at night, suffer from sleep disorders or have certain visual impairments that prevent light from reaching the photosensitive cells of the retina.

how it was discovered

The French astronomer Jean Jacques d’Ortous de Mairan, in the 18th century, was one of the first to raise the hypothesis that living beings have a kind of internal clock that regulates their activities.

He did this from the observation of a plant: the mimosa, popularly known as the opium poppy. Anyone who knows knows that the leaves of this species close when touched or when it gets dark (and they open again when the day dawns).

And what De Mairan realized through experiments was that when he placed the plant in a closed cupboard – that is, in total darkness – its leaves continued to reproduce the same mechanism for several days.

“He saw the leaves of the mimosa plant open and close under constant conditions (darkness, for example),” explains Foster.

In other words, it wasn’t just the light that dictated her rhythm, but something inside her.

But, according to the expert, it was only about 60 years ago that researchers began to study these rhythms in humans.

Knowledge in the area grew to the point that, in 2017, three American scientists shared the Nobel Prize in Medicine for having unraveled “the molecular mechanisms that control the circadian rhythm”.

“So we’ve known a little bit (about the biological clock) for a while, like phenomenology. But the really exciting thing about the last 30 years is that we understand what the biological clock does, how genes and their protein products interact to generate a biological oscillation. 24 hours,” says Foster.