system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes us to wake from sleep if no external force or sound does?
gpt
Basically, we have this internal clock called a Circadian Rhythm. Most organisms have this, and aside from sleep management, it also helps with hormone release and body temperatures. The science behind why it's able to make us wake up, or feel sleepy at certain times is beyond me, but TLDR hormones like melatonin build up after staying awake and gets cleared during the night.