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First of all, we all have a circadian clock than is run endogenously by differential gene expression and controls our change in awakeness, temperature, hormone levels, etc.. Circadian clocks are not exactly 24 h and they vary among people: some people have shorter clocks and other have longer ones. Apparently, people who have shorter circadian rhythms are morning people, and those that have longer one, are night owls.

How is this possible? In the following article "https://www.futurity.org/biological-clocks-sleep-disorders-protein-mutations-2289312-2/" it says that a 20-hour biological clock will make a person fall asleep earlier because they will get tired earlier and they will get awake also earlier. But for me, that would only be possible the first period or two if both cycles start at the same point time. Afterward, the cycles will get out of phase and that will not be true any longer.

graph comparing a long and a short sleep cycle

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Circadian rhythms are entrained by light via the suprachiasmatic nucleus, a part of the brain that receives signals from special retinal ganglion cells that are directly sensitive to (mainly blue) light.

However, light is not strictly necessary: the internal circadian clock is the result of shifting gene expression that proceeds without outside stimulus. Light is only used to entrain (or "reset") the clock each day.

When people talk about the duration of an individual's circadian rhythm, they are referring to what happens in the absence of a light cycle (that is, without entrainment).

https://en.wikipedia.org/wiki/Circadian_rhythm https://en.wikipedia.org/wiki/Suprachiasmatic_nucleus https://en.wikipedia.org/wiki/Circadian_rhythm_sleep_disorder

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    $\begingroup$ So the clock is reset each day, meaning that both periods will start at the same point every day, and since a morning person has a 20-h cycle then indeed, it will wake up earlier and go to sleep earlier than a longer biological clock, I am saying all that right? $\endgroup$
    – Laura
    Jun 5 '20 at 10:22
  • $\begingroup$ @Laura That's the general idea, yes. However the circadian clock isn't everything and isn't the only factor in people being "morning people" or not. It's an influence rather than deterministic, like most biopsychology. $\endgroup$
    – Bryan Krause
    Jun 5 '20 at 14:29
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The Circadian Rhythms of humans is reset by light exposure cycles, but for a cycle to be reset it has to exist first.

Without light there are cycles created by protein production and gene inhibition.

In the relevant 24 cycle there is a is a gene complex that produces a set of proteins that react and decay into a product shuts down the gene's expression (protein production) as long as it is present. that stops the initial proteins production thus creating a cycle based on the proteins reaction and decay rate. So the gene is active and the protein is produced, over a roughly se amount of time the protein reacts and decays into a protein that shuts down the gene expression (inhibitor), Will say this takes time A, of course the gene was till produce the protein until the inhibitor stopped it so the reaction and decay continues for time A again until all the inhibitor is used up and the gene starts producing the proteins again. So you get a cycle of expression and inhibitor that resets once every times Ax2.

But as you can guess genes and proteins a small change in the protein can change can change how long it takes to react and decay. This is how you get some people with slightly longer cycles and others with slightly shorter cycles.

https://pubs.niaaa.nih.gov/publications/arh25-2/85-93.htm

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