Deep insights into your internal clock

What is the circadian rhythm?

The circadian rhythm, also called the internal clock, is crucial to our daily life. It governs our sleep–wake cycle and is strongly influenced by environmental factors, particularly light. When it gets dark in the evening, our body begins to produce melatonin, which makes us feel sleepy. When it becomes light, melatonin production stops and we wake up. This mechanism runs automatically in the background, like a clock.

Interestingly, the circadian rhythm affects not only our sleep but also other physiological processes such as the regulation of body temperature. During the rest phase, body temperature drops slightly and rises again in the early morning hours. This natural temperature pattern supports our sleep–wake rhythm and helps improve sleep quality.

In nature, the circadian rhythm is observed in many organisms that time their activities according to the time of day. Flowers close their blooms at dusk and nocturnal animals only become active at twilight. This illustrates the universal importance of this internal clock, which not only controls our daily routine but also enables harmonious coexistence with other living beings.

How is the circadian rhythm controlled?

The circadian rhythm is controlled by a complex system of biological clocks in the body. These clocks are made up of protein molecules that communicate within cells and are present in almost all tissues and organs.

In the brain there is a central internal clock, the suprachiasmatic nucleus (SCN), which synchronises these biological clocks. It is located in the hypothalamus and functions as a control centre. The SCN coordinates various physiological processes depending on the light and dark conditions in the environment. It receives direct information from the eyes, transmitted via the optic nerve, about lighting conditions. This information regulates the production of the sleep-promoting hormone melatonin. When it is dark or when you are in a dark environment, the SCN signals the brain to produce more melatonin.

Through these mechanisms the suprachiasmatic nucleus (SCN) ensures that our system is aligned with the day–night cycle and that the circadian rhythm functions correctly.

Do babies have a circadian rhythm?

Babies develop their daily rhythm only in the first months after birth. During this period their sleep patterns can be very irregular, as they adapt to their surroundings and undergo physiological changes. From the third month of life infants begin to produce melatonin, and cortisol is produced between the second and ninth month of life.

Once these biological processes have matured, infants and children have a relatively stable sleep–wake rhythm. This regulated daily rhythm results in a sleep need of around nine to ten hours per night.

What does the circadian rhythm influence besides sleep?

The circadian rhythm influences far more than just our sleep. These internal 24-hour cycles play a central role in almost all body systems.

One of the most important influences of the circadian rhythm is metabolism. It regulates blood sugar and cholesterol levels, which directly affect body weight and overall metabolism. In addition, the circadian rhythm has significant effects on mental health, influencing the risk of psychiatric conditions such as depression and bipolar disorder, as well as neurodegenerative diseases like dementia.

Moreover, the circadian rhythm is critical for the immune system and DNA repair, which are important for cancer prevention.

What circadian rhythm disorders exist?

Disorders of the circadian rhythm occur when the internal biological clock (endogenous sleep–wake rhythm) and the external light–dark cycle are not synchronised. This can be due to internal causes (e.g. delayed or advanced sleep phase disorder) or external influences (e.g. jet lag or shift work).

With external causes, other circadian rhythms in the body, including body temperature and hormone secretion, can also fall out of sync. This desynchronisation can lead to symptoms such as insomnia, excessive fatigue, nausea, general discomfort, irritability and depression.

In addition, the risk of cardiovascular disease and metabolic disorders may increase. Adapting to repeated shifts in the circadian rhythm, such as those experienced during frequent long‑distance travel or shift work, is particularly challenging. The adjustment becomes even more complicated when shift rotations occur counterclockwise, for example when travelling east or switching from day to night shifts. Symptoms may disappear after a few days, but in some people, especially older adults, they may persist for weeks or months until rhythms readjust.

Can fasting improve your circadian rhythm?

Yes, fasting can positively influence the circadian rhythm. Intermittent fasting, particularly the 16:8 method, can help support the natural sleep–wake rhythm. This involves avoiding food two to four hours before bedtime so that the body can optimally prepare for night‑time rest.

Food intake is an important factor in regulating the circadian rhythm. When we eat late in the evening, insulin levels rise and digestion is stimulated, even though the body should be preparing for sleep. This can disrupt the liver's night‑time cleaning processes and impair sleep quality.

Unlike other fasting regimens that aim at calorie reduction, intermittent fasting is intended to support the circadian rhythm by better aligning meal times with the natural daily cycle. Our internal clock is governed by external cues such as light, darkness and mealtimes. A late meal can interrupt these essential processes.

How to maintain a healthy circadian rhythm: useful tips

• Keep to a regular sleep schedule. Go to bed and wake up at the same time every day to stabilise your sleep–wake rhythm.
• Optimise your sleep duration to 7–9 hours and avoid frequent changes in your sleep–wake pattern. Your body needs stable regulation of the clocks in the brain for a healthy rhythm.
• When travelling across time zones you may experience jet lag. It is advisable to prepare for time shifts, as they can challenge the human body. It can be helpful to gradually adjust sleep and meal times a few days before travel so the body can adapt more gently and reduce jet‑lag effects.
• Use natural time cues such as light and meals. Start the day with daylight, especially early in the morning, to support your circadian rhythm.
• Help your internal clock actively by being physically active during the day so you can fall asleep more easily in the evening.
• Limit caffeine intake after lunch to avoid disturbing your natural sleep–wake rhythm.
• Reduce exposure to artificial light before bedtime, especially from electronic devices, to avoid disturbing the clocks in the brain. This can improve sleep duration, reduce sleep debt, decrease nocturnal awakenings related to sleep disorders and prevent rhythm disturbances.
• Short naps (20–30 minutes) can be refreshing, while longer breaks can interfere with night‑time sleep.
• Plan your working hours carefully. Switching between different shifts can disrupt the circadian rhythm. Shift length, rotation and sleep deprivation should be considered to avoid rhythm and sleep disorders. These effects are highly relevant for human health.
• Avoid heavy meals late in the evening and keep your dinner light and nutritious.
• Do something relaxing before bedtime. Reading, meditation or a warm bath can help prepare the mind for sleep.
• Ensure a comfortable sleeping environment. Keep the bedroom cool, quiet and well ventilated.

The circadian rhythm is a natural order by which the body adapts to the day–night cycle and thereby supports a healthy sleep–wake rhythm. Regulating the internal clock and sleep cycles is necessary to prevent sleep disorders. An active lifestyle with sufficient recovery periods is essential to maintain this vital function. A mindful approach to this rhythm can help promote overall well‑being and reduce potential health problems.