Internal Clock

Some things in life aren’t certain, but one thing is sure: the sun will rise every morning and set at night. The Earth has turned in a steady 24-hour rhythm for the past 4,5 billion years. Most life forms have adjusted to the day-nigh-rhythm, and to do so, they have evolved an inner clock to handle the changes in light during the day and night. With this biological clock, they can predict and guide their vital functions according to the time of the day. For example, many plants can predict the sunrise beforehand and open their leaves just in time to get the maximum benefit of the sunlight.

In humans, the internal clock (or circadian rhythm) optimizes bodily functions, such as body temperature, metabolism, and hormonal processes, based on the time of the day. When working correctly, your inner clock makes you sleepy when it’s time to go to bed in the nighttime and increases your alertness and performance during the day. Melatonin (“the hormone of darkness”) is the hormone, primarily in charge of the time we fall asleep. The circadian rhythm is heavily controlling its secretion. Melatonin starts to secrete in the nighttime when the day gets darker, and its message to the body is clear: the night is coming, time to go to sleep. The amount of melatonin decreases during the night, and the secretion stops entirely in the morning before waking up. This time the message is, correspondingly: the sun is rising, time to get up.

Our genetics determines the length of the 24-hour cycle. However, it needs outside help to stay aligned with the outside world. The most important external pacemaker for the internal clock is light. The human body works in its best when the inner clock is in sync with the day-night rhythm in the environment. That’s why it is highly essential to get as much light as possible in the morning and equally important to avoid artificial lighting in the evening and dim the lights several hours before bedtime.

Habits From This Lesson

Additional Reading


Borbély, A. A., Daan, S., Wirz-Justice, A., & Deboer, T. (2016). The two-process model of sleep regulation: A reappraisal. Journal of Sleep Research, 25 (2), 131-143.

Panda, S. (2018). The Circadian Code: Lose Weight, Supercharge Your Energy, and Transform Your Health from Morning to Midnight. Rodale Books.

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