"Lying Girl in the Greenery" by Georg Schrimpf (1889-1938). STAATSGALERIE STUTTGART/BPK/RMN-GRAND PALAIS

It is all about our genes and the way their activity oscillates within our cells during the day and night. These oscillations form two large waves, one in the morning and the other in the evening, with varying levels according to the tissue. This also impacts the gene cluster involved. Moreover, these waves appear more marked in women than in men, but in both sexes, they are dulled by age.

Essentially, these are the findings of a groundbreaking exploration conducted on human tissue taken from 914 donors shortly after their death. Published in the February 3 issue of the journal Science, this analysis brings to light the intimate mechanics of our circadian clock, the molecular "gearbox" that gives our cells the time of day – so they can adjust their tasks to the body’s needs.

But what about the name "circadian" clock – from the Latin circa, "near," and dies, "day"? For humans, this clock is set over an average period of 24 hours and 12 minutes. However, this duration depends on the individual. In early sleepers, the clock is faster (its period is more in the region of 23 hours and 30 minutes); in late sleepers, it is slower (24 hours and 30 minutes). Whatever our chronotype, our internal clock must be set to a 24-hour rhythm. To do this, it uses external synchronizers, namely daylight, the timing of our meals and our daily activities.

Read more Why are women more sensitive to the cold than men?

Diseases due to the clock's disruption

Almost all of our body's functions are subject to this 24-hour rhythm, such as our sleep-wake cycle, blood pressure, heart rate, body temperature, appetite, mood, cognitive abilities, immune responses, cell division and DNA repair. When this clock is disrupted, various diseases can result or worsen, including diabetes, cancer, cardiovascular diseases and sleep disorders.

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The mechanics of our internal clock works like a gearbox containing about 15 known genes. "Each of them is activated at a specific phase of the day," said Felix Naef of the Swiss Federal Institute of Technology in Lausanne (EPFL), who coordinated the study. For example, the PER3 gene peaks in activity around 9 am and the BMAL1 gene around 9 pm. For the NR1D2 gene, this peak occurs around 7:30 am and for CRY1 around 6 pm. This timeline was established in 2018 based on muscle biopsies taken at different times of the day from living individuals. However, this invasive method would be impossible to apply on a wider scale to all tissues.

The strength of the new work lies in the development of an algorithm that gives the tissue's internal clock time, from a single sample. It is done by looking at the activity levels within that tissue, of the 15 or so genes that define this clock.

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