What is the biological basis of our circadian rhythms and how is it relevant to our lifestyle?

What is the biological basis of our circadian rhythms and how is it relevant to our lifestyle?

The cycle of sleep and wakefulness is perhaps the most important for human behavior as we spend a third of our lives by sleeping and cannot exist without it.

As we sleep our brain proceeds to filter out the information that we received throughout the day and our organism restores itself, gets rid of toxins and allows us to function normally when we wake up. Therefore, even a short-term lack of sleep significantly affects our well-being. The alternation of sleep and wakefulness is a necessary condition for the life of the human body. People have been trying to understand what sleep and dreams are since ancient times. Currently, dozens, if not hundreds, of institutions around the world are engaged in sleep research, using the latest advances in science and technology. However, much about the nature of sleep is not yet known.

Sleep is a natural, periodic state, that involves reduced response to environmental stimuli and decreased mobility ("Definition of Sleep", 2022). The course of sleep is usually controlled by 2 processes:

1. Homeostatic (S) – meaning that if we do not get enough sleep, we will accumulate sleep debt and make up for it next time we go to sleep.
2. Circadian - meaning that sleep tends to happen in a particular time during 24 hours cycle. Humans are diurnal, which means that we have a tendency to be awake during the day and fall asleep at night.

There are natural processes within our brain - secretion of neurochemicals and hormones that induce sleep and wakefulness cycles. The main hormone is called melatonin, which is secreted by the pineal gland during dark and promotes sleepiness, however it does not work in isolation. For instance, adenosine – a chemical compound found in human cells, that accumulates during the day after prologue wakefulness and promotes sleep.

To conclude, many chemicals in the brain are involved in promoting sleep/arousal and these are mutually inhibitory (both actively inhibit one another).        

There is another aspect of sleep that was mentioned before - circadian rhythms. Some may know that circadian rhythms are associated with sleep, but sleep regulation is not the only task of circadian rhythms. Everything depends on them – immune system, blood pressure, hormones and even the aging process. The circadian rhythm is the human biological clock that follows a 24-hour cycle. These rhythms usually respond to the light and dark and therefore affect most species on earth. ("Circadian Rhythms", n.d.).

To expand more, these circadian rhythms are really endogenous cycles, so it means that they are created from within our brains. This 24-hour rhythms not only control sleep and wakefulness, but also other important functions such as: eating, drinking, body temperature, secretion of hormones, urination and so on. Circadian rhythms as a conductor controls the whole orchestra of tasks in our body, and if we ignore our cycles, disrupt sleep patterns, then health weakens.

Circadian rhythms are maintained under conditions of isolation of light or darkness. This was discovered as a result of an experiment conducted in 1729 by the French scientist Jean-Jacques de Meran (Foster, 2016). The essence of the experiment was that the scientist placed the plant in a dark place. After making observations, he noticed that even in constant darkness, the leaves open and close in the same rhythm. This was the first evidence that circadian rhythms have an internal origin. They can fluctuate and, depending on the species, be slightly longer or shorter than 24 hours. (Foster, 2016).

Following that, in 1972 two independent laboratories (Moore & Eichler and Stephan & Zucker) carried out that idea and have pinpointed that the main mechanism of the biological clock is the suprachiasmatic nucleus (SCN) of the hypothalamus. This part of the brain is responsible for the response to stress, sexual behavior, coordination of actions and other important functions. The suprachiasmatic nucleus also tells the pineal and endocrine glands when it is time to produce the sleep hormone melatonin. Melatonin levels increase in the evening, peak at night, and decrease during the day. And moreover, with another important hormone, cortisol, which is a stress hormone that works opposite to melatonin. Its concentration rises in the morning, when we need to be alert, and decreases in the evening, when we begin to feel sleepy.