system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do our bodies know when it’s time to wake up?

I’m aware that sunlight makes our bodies produce cortisol, but what causes us to wake up when sunlight isn’t a factor?
gpt
We have a natural "Circadian Rythm". It's 25 hours long though, and needs sunlight to correct itself occasionally. In an enviroment without sunlight it will quickly go out of sync with the actual time of the day, but you will still sleep about once per day.