You are a clock.
Not just you, your dog, the cat, the fly on the wall.
Studying that clock seems to have a lot to do with jet-lag:
https://www.scientificamerican.com/a...iological-clo/
Seem obvious; I sleep, I wake up...repeat; but figuring it all out was worth a Noble Prize:
https://www.scientificamerican.com/a...-researchers1/
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