system
Given the following post, respond as if you're commenting on Reddit:
human
When we are young, we pee the bed.  Why and what causes us to stop doing that?

I was just thinking about how I sleep and I'll wake up in the middle of the night to pee sometimes. I don't know exactly what wakes me? (Without any looking into it, I assume that at the end of an REM cycle, your body does a self check and wakes you up if you have a full bladder, etc).  That eventually led to the thought process of wondering why children eventually "grow out" of it?
gpt
In the urethra of a human we have 2 sphincters. One of which is under our voluntary muscle control. When we are younger we don't quite have complete mastery of "holding it in." As they get older they're able to hold it even when they sleep.