system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why, after a certain point of being tired, do we switch and get energized instead?

Or well, not really "energized" but after staying up for long enough, why is it significantly harder to fall asleep?
gpt
The longer you stay awake the more stressed your body gets. Now to counter the effects of sleep deprevation your body is flooded with adrennaline to keep you awake. At some point you are practically so flooded with adrennaline that you can't sleep. Thats also why you sometimes can have a racing heartbeat while trying to sleep.