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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.

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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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