What is an internal clock? Watches determine our lives. They follow us wherever we go, whether on our wrists, smartphones, laptops, office walls or church steeples. Hardly any social process would be possible without a timepiece – or so we believe. Few realize that humans are the only living beings that would hardly be able to survive as a species without this artificial timekeeping.
Externally, we have therefore made ourselves largely dependent on the measurement of time by aligning our day/night rhythm with the clock. But what does it look like inside us?
The clock that doesn’t tick
Today, very few people think about how they “tick” when detached from man-made time. What’s more, they can no longer feel it at all. But it really does exist, the internal clock. Unfortunately, the term was poorly chosen, because strictly speaking it is not a clock that measures time, but a rhythmically acting pulse generator. Because, and this is an important fact, nature knows no time. She only knows rhythms. All (and by “all” I mean “all” in the truest sense of the word) natural processes are subject to a rhythmic organization. Processes that happen so quickly that we do not perceive them (e.g. ultrasound) or so slowly that we do not perceive them either (e.g. birth, life and death of planets or solar systems). In the same way, all our processes are rhythmically organized. A central driver plays an important role here. The internal clock.
What makes us tick
This internal clock is the pulse generator for millions of other internal clocks in our body, right down to every single cell. This central internal clock, also known as the master clock, is located directly above the optic nerve junction in the ventral hypothalamus and is called the “suprachiasmatic nucleus” or “suprachiasmatic nucleus” (SCN for short). This in turn sends impulses to other “internal clocks”, which in turn provide impulses for other internal clocks. In principle, the process is comparable to the chain reactions of dominoes. The central impulse (in this case the human being) comes from the human being (similar to the SCN). The falling first tile then forms the impulse for the next tile and so on.
What makes us tick
The important thing to know is that each SCN is structured differently. Just as our genes give each person a different foot size (we cannot choose or change this), they also give us differently structured SCNs. How an SCN ticks, whether it “makes” us an early type or a late type, has therefore been written into its code by the genes. And just as we with size 42 feet do not fit into size 39 shoes, late types do not fit into the time structures of early types. Our internal clock therefore plays a key role in determining the position of our sleep-wake rhythm and therefore our chronotype.
The SCN structures, the sun adjusts
The genes and the SCN are the two central elements that make up our internal clock. But there is also an external factor that has an influence on how we tick – the sun. The Andechs bunker experiments have shown that the sleep-wake rhythm based on the SCN is different for each person, both in terms of its position and its characteristics. Physiologically, for example, the internal clock of early types (larks) has an intrinsic period of slightly less than 24 hours, which makes them get tired earlier in the evening and wake up earlier in the morning (usually without an alarm clock). Late types have a natural period of slightly more than 24 hours, which makes them fall asleep later and wake up later (usually with an alarm clock), which also shows that nature does not know exact times to organize itself.
In order to keep the different structures of the internal clock viable, the SCN uses one of the most reliable pulse generators on earth, namely the sun. Their light adjusts the individual SCNs of all people, provided they have access to sunlight, similar to radio-controlled clocks, which are repeatedly adjusted by central atomic clocks.
It is therefore the interaction between genes, SCN and sunlight that ultimately determines our internal clock.
You can find more information on chronotypes here
