The internal Clocks and why they are called Circadian

The oscillating world within us

A myriad of rhythmic processes is surrounding us and happening inside of us, having periods ranging from milliseconds to several years. We live in a constantly oscillating world!


The cyclic behavior of our planet, the 24-hour period of Earth’s rotation on its axis, brings us the regular day-night transitions in illumination, temperature, and more. To survive and succeed, all living creatures had to adapt to this periodic change in their environment.


How do you know when to be ready for something? By looking at your clock or watch, or phone. In the early days, like hundreds of millions years ago, lack of watches and phones led to molecular creativity. Each cell, from a bacteria to those forming your body, got itself a Rolex, a luxurious internal molecular Clock.


Those Clocks turned out to be so handy and so critically important for making things right for our cells that they are diligently using their own Clocks up to this day. The inner Clocks predict when the Sun goes up or down, and thus prepare cells way in advance for the daytime or nighttime internal processes planned.


These molecular Clocks are cell-autonomous, meaning that the machinery to run the Clock fits within a tiny single cell, serving the cell's internal purposes. Yet, the Clocks are also critically important for the cells to live well together in our multi-cellular organism. Easier said than done, considering that there are around 37 Trillion individual cells making up each of us!


Like factory workers, cells align themselves to work the assembly lines of numerous molecular processes. Success of the entire organism is a result of cells' team work. Hence, each cell has to do all it is supposed to do on time. If cells follow their Clocks, our health, cognitive and physical performance, sleep, even the way we look, are at their best.


The role of the intrinsic Clocks in our health and disease turned out to be so important that the discovery of the Clocks' molecular mechanisms was recognized by the 2017 Nobel Prize.

The 2017 Nobel Prize in Physiology was awarded for the investigation of the molecular mechanisms controlling the circadian rhythm.
The 2017 Nobel Prize in Physiology was awarded for the investigation of the molecular mechanisms controlling the circadian rhythm.

Free-running Circadian Rhythms

Our inner Clocks are self-sustained oscillators. They keep ticking under constant conditions, when there is no periodic change in environment and no external time cues.


This was first documented in plants, then animals and humans, by placing them in constant dim light or darkness for many days. All these living creatures continued to show distinct rhythms of activity, hormonal or metabolic processes.


Surprisingly, they did not follow the exact 24-h period of our planet. Turns out, they were free to choose their own internal period within a certain range and they did. That is why these stable self-defined internal rhythms were called the free-running rhythms. Their typical period ranges between 23h and 25h in normal organisms.


Because the internal free-running Clocks display periods close but not equal to 24h, they were called Circadian Clocks, from the Latin circa = around, and diem = day. Accordingly, the rhythms they produce were called the Circadian Rhythms.


Almost all our body functions, physical or mental, demonstrate Circadian Rhythms. Just to name a few: sleep-wake cycle, metabolic and hormonal cycles, cognitive, mood and cardiovascular cycles, physical strength and immune response cycles, and many more.

About Irina Zhdanova

Irina is the CEO and founder of ClockCoach, and an expert in Sleep Medicine and Circadian Neuroscience