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The circadian cycle, also known as the circadian rhythm, is the 24-hour period in which the internal biological clock maintains the body's activities and biological processes such as metabolism, sleep and wakefulness, and is influenced by exposure to different types of brightness throughout the day.
This occurs because the brain receives different stimuli when it is day or night, producing hormones such as cortisol and melatonin, altering body temperature and regulating metabolism to keep the person awake or asleep.
Some factors can disturb the circadian cycle, such as sleeping late, working or eating hours, having insomnia, traveling that involves time zone changes and even the beginning or end of daylight saving time, leading to dysregulation of the biological clock and the appearance of symptoms such as excessive tiredness, loss of concentration, headache or irritability.
Furthermore, each person has their own biological clock and that is why people are classified as morning, afternoon or intermediate, according to the periods of sleep and wakefulness they present 24 hours a day. Learn how to identify your own biological clock.
Physiology of the human circadian cycle
The human circadian cycle is controlled by the hypothalamus, a region of the brain that receives signals about light and dark, picked up by the retina in the eyes, and establishes sleep and wake patterns throughout the day.
These signals are transmitted by the hypothalamus to the pituitary gland in the brain, which is responsible for producing a hormone called melatonin, which in response to darkness has its levels increased to prepare the body for sleep, lowering body temperature, blood pressure, heart rate, breathing, body metabolism and urinary system activity.
During the day, when the retina detects light, melatonin production is inhibited and the brain sends stimuli to the adrenal glands to increase cortisol production to make the body more alert and increase wakefulness during the day. This hormone can also increase in times of stress or be higher in chronic conditions, which can compromise the proper functioning of the circadian cycle. See what the hormone cortisol is for.
Circadian rhythm disorders
The circadian cycle is driven by the change in luminosity from the darkness of night to the brightness of the day, regulating the body's hormone levels and the state of sleep and wakefulness.
Some factors can contribute to altering the circadian rhythm such as working at night, pregnancy, time zone changes, medication use, routine changes such as staying up or sleeping late, menopause, and even diseases such as Alzheimer's or Parkinson's, for example.In this way, not only sleep is affected, but it can also cause disturbances in hormone production, changes in body temperature, metabolism and eating habits, which can lead to the development of diabetes, obesity and depression. Learn more about circadian cycle changes and how to treat them.