How do circadian cycles influence health?

A summary of the ways in which circadian rhythms affect our health status.

In biology, circadian rhythms are defined as oscillations of an organism's physiological variables at regular time intervals. All living beings, from bacteria to humans, show oscillations in our parameters depending on environmental conditions, which tend to change rhythmically and in step with our body.

In our species, circadian rhythms encompass physical, mental and behavioral changes that follow a 24-hour rhythm. Most of these mechanisms are based on light-dark variation, because for humans, nighttime involves rest and repair, while daytime is a peak of physiological and neuronal activity. As you can imagine, not all living beings respond in the same way to the light-dark cycle, just as they do not interpret other environmental parameters such as dryness, heat, rain and many others in the same way.

Circadian cycles are essential for the health of human beings and all animals, no matter how independent we may seem to be of them.They are essential for the health of humans and all animals, no matter how independent we may seem from nature in an increasingly anthropized Earth. If you want to know how the circadian cycles of our species influence our health, read on.

Circadian rhythms and the Biological clock.

Although biological clock and circadian rhythm/cycle are widely linked terms, they do not mean the same thing in either case. in neither case. Rhythms are physical, mental and behavioral changes that we carry out in a cyclical manner every 24 hours, while biological clocks are the mechanisms we present within ourselves to regulate the cycle of circadian rhythms.

In mammals, the biological clock is located in a specific area of the brain, the suprachiasmatic nucleus.. This consists of a cluster of about 20,000 neurons in the medial hypothalamus, which integrate the cycles based on the concentration of the CLOCK and BMAL1 proteins, among other things. We will not go into complex terminology, since it is enough to know that the genes encoding these proteins allow their dimerization, which activates genes such as period (per) and timeless (tim), essential in the mechanisms described here.

How do these cycles influence health?

We have seen a few hints about the biological clock of living beings, but this mechanism stands out for its complexity and nuances. In the following, we explore the effects of the imbalance and maintenance of circadian rhythms on health.

1. The relationship of circadian rhythms with diet

As we have seen above, the biological clock tends to keep pace with the rhythms of light and dark in the environment. Multiple sources state that nutrient intake can modulate the expression of certain circadian rhythm regulator genes, which alter feeding behavior in the hypothalamus.. These concepts fall into the field of epigenetics, i.e., the existence of changes in gene expression/inhibition without disrupting the genome itself, but in response to environmental factors.

An interesting article that investigates these correlations is Impact of nutrients on circadian rhythmicitypublished in 2015 in the scientific journal Regulatory, integrative and comparative physiology. This review article cites that, indeed, diets high in fat and sugars can lead to alterations in genes that modulate our biological clock, which translates into effects on peripheral tissue and the central nervous system (CNS).

However, some sources argue that, in order to reset the biological clock, the total amount of nutrients (energy) is more important than the nature of the nutrients. Much research is still needed, as we are in the infancy of epigenetics.

2. Melatonin and sleep

Melatonin is the most important hormone in living beings when it comes to the regulation of the sleep-wake cycle.. The suprachiasmatic nucleus receives information from external light from the eyes and, in times of darkness, signals the pineal gland to produce melatonin. The increased concentration of circulating melatonin subconsciously signals us that it is time to go to sleep.

To date, it has been shown that exposure to bright lights at night prevents melatonin secretion, which delays normal circadian cycles. The study Self-luminous devices and melatonin suppression in adolescents demonstrates this in a simple way. In this research, a group of adolescents were fitted with special glasses (which cancelled out the incidence of light from electronic devices) before bedtime, while others maintained their normal routine without glasses. Melatonin levels were measured in both experimental sectors throughout the nights.

Children who did not wear glasses had 28% less circulating melatonin than those who did after one hour of exposure to a screen before bedtime. Based on data such as these, it has been estimated that prolonged exposure to a tablet or cell phone before bedtime can delay sleep by more than an hour..

3. Circadian cycles and body temperature

Circadian rhythms and the biological clock also condition the temperature we present inside our body throughout the day. Individual body temperature decreases during the night (time of reduced metabolism), reaching its minimum at 3 am and its maximum at 6 pm.. Thus, between 6:00 a.m. and 6:00 p.m., an increase of approximately 0.5 degrees is expected.

Therefore, a person should not worry if his or her body temperature is lower than normal at night, especially between 2 and 4 am. Even so, any temperature below 35 degrees is considered hypothermia, of greater or lesser severity on a case-by-case basis.

Summary

As you can see, circadian cycles modulate our hunger and appetite for food, our body temperature, the rhythm of our sleep and much, much more. Although the biological clock is encoded in our genetic makeup, there are a number of factors and habits that can tip its functioning to a more positive and natural balance. The amount of circulating melatonin and exposure to screens are a clear example of this.

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Bibliographic references:

• Figueiro, M., & Overington, D. (2016). Self-luminous devices and melatonin suppression in adolescents. Lighting Research & Technology, 48(8), 966-975.
• Oosterman, J. E., Kalsbeek, A., la Fleur, S. E., & Belsham, D. D. (2015). Impact of nutrients on circadian rhythmicity. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 308(5), R337-R350.
• Reddy, S., Reddy, V., & Sharma, S. (2020). Physiology, circadian rhythm. StatPearls [Internet].