system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes me to wake up minutes before my alarm goes off, even when I've barely slept?

For example, I woke up at 5:45 am today, like I would normally do to go to work, except I had the day off. I went to bed at 2:30 am, so it's not like I was fully rested either. What causes this to happen?
gpt
There is something in our body called a biological clock. If your body is used to doing some things at a certain time, be it waking up or maybe using the restroom. You will generally do these things at those times even when its pointless to do so otherwise. Since you work your body does this to help you not oversleep when its time for you to work. 

I hope this helps you.