CIRCADIAN RHYTHM
THE BODY'S 24-HOUR CYCLE

Circadian rhythm

We feel full of energy and vigor when the sun is shining and tired as soon as it gets dark. That’s because the human body has the ability to adapt its biological functions in a 24-hour cycle. We are already programmed genetically to follow a circadian rhythm. This rhythm allows us to perform recurrent actions like eating, sleeping or regenerative phases in a regular cycle and to adapt our bodily functions accordingly. This article tells you all you need to know about the driving force behind this rhythm – the body's internal clock.

The physiology of the circadian rhythm
The word circadian comes from Latin and roughly translated means "around a day". Back in the 16th century, Carl von Linne described a flower clock, with the flowers of certain plants opening their flowers at different times of the day. The French physician Virey discovered in 1814 that the bodily functions of living beings change over the course of the day. These were the beginnings of chronobiology. Internal clocks could be proven to exist in practically all living things, even in bacteria and fungi. The most clearly discernible circadian rhythm is the sleep-wake cycle. In addition, however, there are numerous other cycles that change during the day, such as hormone secretion, metabolism or body temperature.

The internal clock is subject to many influences:

  • A genetically predefined rhythm, which takes around 24 hours, but is somewhat faster in some people and slower in others, is the underlying basis.
  • Light as an internal clock, in other words light that falls on a particular type of light-sensitive cells in the retina of the eye and triggers signals there, which are passed directly to the brain’s control center for biological rhythm. Brightness stimulates these photosensitive ganglion cells in the eye, telling the body that it is daytime. The internal clock is set to "start" in the morning and regulates the daily routine. Light with shorter wavelengths and a high blue content, which occurs primarily in natural daylight, is especially effective in this regard.
  • Melatonin acts as a sleep hormone in people. It is only emitted into the blood from the pineal gland in the diencephalon during darkness, thus regulating the day-night rhythm by making us tired and sleepy. Melatonin enters every cell in the body via the blood and conveys the information there that it is nighttime.
  • Social factors can sometimes influence the circadian rhythm negatively. If you regularly have to get up in the morning when the internal clock is not yet set to daytime, or have to work in the evenings when the body demands its sleep, this can have a negative health impact in the long term. If you have your days free after working a night shift, but your internal clock will not allow you to sleep because it is set to daytime activity, the body experiences what is referred to as social jet lag.

According to the latest studies, practically all organs in the body have specific internal clocks. In the early hours of the morning before we wake up, the body receives initials signals that it is time to start the day. Hormone production is stimulated, the heart rate increases and blood pressure rises.

The body is already preparing itself for the day ahead. Intestinal activity is at its strongest around 8:30 am. You are wide awake and alert by 10:00 am. Coordination and responsiveness reach their peak by early afternoon. Cardiac activity and physical fitness are at their highest around 5:00 pm. This is the best time for peak sporting performance. The discharge of melatonin into the bloodstream starts in the evening around 9:00 pm, making us feel tired. Metabolism drops while the immune system is working actively during sleep. This is also when hair growth is at its strongest. This rhythm should not be disrupted if possible.

Consequences of a disrupted internal clock
The body is genetically programmed to maintain a time balance between environment and organism, but needs to be synchronized each day with the progression of the day. The simplest way to do this is through natural daylight. If this balance is disrupted, for example because of a lack of light, sleeping and eating patterns are typically disrupted, which can lead to mood swings, daytime fatigue and depression. External factors are usually the reason why the biological rhythm gets confused – lack of light in the winter or too much light in the evening from computers, TVs or mobile phones can all contribute to this. Studies also show that the risk of serious illnesses, such as cancer, heart attack or diabetes, increases as a result of a permanently disrupted circadian rhythm. Those affected are also prone to severe obesity.

The right light at the right time
The body's circadian rhythm can be influenced positively, with the main determining factor in this respect being the right light. But this can often be a scarce commodity in the winter months especially. Bright artificial light with a high blue content, similar to natural daylight, can help in this case to stabilize the internal clock in the morning and provide the necessary stimulation during the day in order to be active and productive. And to ensure that this effect does not prevent you from sleeping at night, more subdued lighting with a lower blue content should be chosen in the evening hours. Tip: Dim the light, use warm white light colors (2700 K) and simply put the mobile phone or tablet aside around two hours before going to bed.