system
Given the following post, respond as if you're commenting on Reddit:
human
What makes us wake up after we sleep?
gpt
Circadian rhythm. It prefers to go to bed and wake up at a routine time because your body is more efficient that way. It's also effected by the amount of light around you, which is why some people can stay up really late starring at a computer screen or their cell phone, because they are simulating light.  
Also there was a study shown that if someone anticipating waking up early the next day they would start making stress hormones an hour or two before they woke up which allowed them to wake up less groggy. Those who didn't expect to wake up didn't produce those hormones and woke up very tired.