system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - Why if you stay up for 24+ hours your body sometimes doesnt let you fall asleep when you want too

You would think that the longer you stay up the easier it is to go to sleep after but that’s not always the case, and i would love to know why! thank you!
gpt
Your body and most living thing have something we call a circadian clock. It's an biochemical oscillator that synchronise with the sunlight and other external variable. This make your body create a circadian rhythm or 24 hours and regulate your sleeping schedule.

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So if you force yourself to no sleep, you body gonna keep sending signal for your to sleep up to a certain point when your body gonna stop, since by that point you are suppose to wake up and stay up according to your internal clock. Your clock doesn't really know that you didn't sleep, it just know that at this point you should be awake. So usually, you don't really feel sleepy for the rest of the day, until it's time to go to sleep again.