Clocks, internal time and cardiometabolic health in the context of the hour change
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Imagem de um despertador e uma mulher a dormir

Again, when entering “summer time”, the contours of life also change. Life is as old as biology and has succeeded thanks to success in overcoming time and barriers. Everywhere, the virtue of measuring time has been recognized by the machine of evolution. In each of their aspects, the clocks were perfected according to individual and collective particularities, in view of the desirable interaction on a “social” planet.

This socialization integrates dynamic time values, such as light, temperature, humidity, and others, which, more or less volatile, obey an assertiveness dictated by physical and chemical impositions that the movement of the universe does not let out of its control. The rotation of the earth over itself results in a cycle of light-darkness, which is completed every 24 hours. From its translation and the inclination with the central axis, years and seasons are born. Inherently, the time of rain and drought, cold and heat varies.

This has been the case throughout evolution. It was this ability to anticipate time and the events that depend on it, that allowed us to build, plan and survive... sleeping, for example. And although it is in sleep that the most attentive eyes are projected to the influence of circadian dysrhythmias in the body, one of the most important resulting from the malfunction of the clocks, it is anarchic cell proliferation that characterizes the development of many malignant tumours. Programmed cell death, one of the biological assumptions that protects this occurrence, is “silenced” by changes in the internal temporal system. In fact, some of these changes were confirmed in an experimental process simulating the change in the legal time. But whereas the reserves of in vitro, molecular and biochemical studies, may raise validation doubts as to their applicability in real life, other studies of a clinical or epidemiological nature, pragmatic and more easily to extrapolate, have rejected the null hypothesis about the absence of differences in the periods after the time changes when compared to other periods at different times of the year. One of the aspects that had the greatest impact was the increased number of cardiovascular episodes after the change in daylight saving time. Studies by our group showed that happened in Portugal. In 2019, we confirmed a greater number of cardiovascular and metabolic events in the week after the time changed and that it was statistically different from the rest of the year. In the scientific literature, there are a dozen studies which, although with methodological differences, corroborate these results, showing, for example, that the increased risk of heart attack after the time change varies between 4% and 29%. These figures, of course, include the influence of chronic sleep deprivation and the "Monday effect", both factors that enhance circadian misalignment. In the opposite direction, it is certain that the breakdown of cell clocks that regulate the sleep-wake cycle leads to sleep difficulties. In addition, directly or indirectly, the chronotype, a representation of the individual temporal identity, which often translates the midpoint of sleep (and which characterizes people according to morning, evening or intermediate type), is also associated with different risks in terms of cardiovascular and metabolic function. According to studies, evening type persons have greater association with risk factors related to cardiovascular disease, as well as greater propensity for changes in glycaemic control, and for developing metabolic syndrome and diabetes.

It is really easy to imagine the comfort that a social hourly synchronization allows for the movement of financial markets, economic and political-administrative coordination and even physical phenomena explained by the different angles with which the sun, in its trajectory, illuminates the multiple time zones.

The claims about the advantage and the purpose of changing the time regarding winter sun exposure and how it can influence school performance are comfortable, but inconsistent. This argument is confirmed by the French interventionist policy, which a few years ago, repositioned school schedules with unequivocal benefits in cognitive terms. This policy can certainly reach other spectres of work and social life. The difficulties will therefore arise at various levels of the community hierarchy. Even so, respecting the hours we keep in our genetic history, hearts will continue to beat, probably at the right pace, dictated by intact clocks that nature has organized and improved over time. While they work, they can guarantee that time and space are combined with elements fundamental to life, such as well-being and health.

Miguel Meira e Cruz

Director of the Sleep Unit of the Cardiovascular Centre of the University of Lisbon, Faculty of Medicine