The circadian rhythm is controlled by a special part of the brain called the hypothalamus. It processes light signals from the optic nerve and triggers the release of certain messenger molecules that transmit information from one neuron to another. Scientists have identified five such mediators that control our sleep and wake cycles. These are serotonin, dopamine, norepinephrine, acetylcholine, and gamma-aminobutyric acid (GABA). The latter promotes sleep, while the others increase alertness and prevent falling asleep. During wakefulness, the number of these mediator molecules increases significantly. The strongest desire to sleep occurs between two and four o’clock in the morning, when the circadian rhythm peaks, and most people are most active in the afternoon between one and three o’clock, although the exact time may vary depending on individual characteristics. In the morning, the eyes are exposed to intense light signals, which raises body temperature, increases the production of the ‘stress hormone’ cortisol, and delays the release of the ‘sleep hormone’ melatonin. But of course, in conditions of reduced daylight hours in winter, when working in poorly lit rooms and travelling frequently, the main regulatory systems responsible for the sleep-wake cycle become disrupted.

Melatonin production by the epiphysis (endocrine gland) is activated in the dark. That is why experts recommend sleeping with the lights off, hanging thick curtains at home and not using gadgets at night. After release, melatonin moves through the brain to a special cluster of neurons – the suprachiasmatic nucleus – and gets from there into nearby blood vessels to spread to the rest of the body. When levels of this hormone are reduced, we can experience not only seasonal mood swings, but general health setbacks. Not too long ago, a study came out that suggests melatonin is linked to cardiovascular function. Scientists have found that the hormone can have a positive effect on heart function and stabilise blood pressure.

Although sleep is considered an important process, it is still not entirely clear why it is necessary. There are several physiological processes that occur in the brain during sleep that are important for bodily functions. Scientists suggest that sleep, and dreams in particular, are a mechanism for reproducing and processing mental stimuli to extract meaning from them and create memories. Many researchers also believe that sleep is necessary to systematise the physical ‘rubbish’ in the brain that accumulates during a person’s waking hours. Such an example of “rubbish” is beta-amyloid formed from protein molecules, which, according to scientists, over time damages brain tissue and impairs memory in the elderly. As a result, they develop neurodegenerative diseases such as Alzheimer’s. International travel and jet lag can disrupt the natural circadian rhythm and cause sleep difficulties, especially in the first few days when the body is adjusting to the new environment. Sleep disorders are often the subject of clinical research because they accompany almost all types of mental illness. This is why it is so important to monitor your circadian rhythms and adjust the timing, duration of sleep and rest when moving or changing work schedules.