Humans have an internal clock that tells them when to start producing a chemical called melatonin that waked us up. This internal clock is called a circadian rhythm.
Fun fact, certain types of light also cause the production of melatonin. Normally when the sun goes down, that melatonin-producing light isnt there anymore. This is how phone and computer screens can keep us awake past the night part of our circadian rhythm.
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