system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does my body know when to take a 90 minute nap, vs taking an 8 hour nap?
gpt
Human circadian rhythms are on approximately 90 minute cycles. This means when you are asleep you go through progressively deeper stages of sleep, until you have a period of rapid eye movement (REM) sleep, and then the cycle repeats itself over again. Your body knows when the cycle is resetting and goes back to the very light sleep stage. If you ever wake up in the middle of the night feeling like you haven't been sleeping it's probably because you just started a new sleep cycle.