Circadian sleep cycle – congenital factor. Slightly different for each species and their instinctive survival strategies. Something like some BIOS function in a computer.
Imagine that this function – computer’s internal clock. Just because it exists doesn’t mean you can’t install other software clocks, aspects of adaptability of the human body.
By habits or by artificially altered daylight hours you can perceive circadian sleep cycle differently, but it cannot be changed.
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