system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why Is It That I Can Hold My Bladder During 8 Hours Of Sleep?

I am not sure if I'm alone in this but, sometimes before falling asleep, I can feel that my bladder is full, and by full, I mean "I would piss like a race horse if I'd dare to relieve myself" full, and I'm too tired to get up to go to the bathroom, but I can spend the whole night with my barely-exploding bladder and still not wet my bed.

How is it possible that my bladder muscle (not sure if it's an actual thing), keeps all that piss in without me actively trying not to piss meself.
gpt
Because when you fall asleep, you pituitary gland produces the hormone antidiuretic hormone (ADH) which instructs your kidneys to slow down the production of urine. This prevents your bladder from getting too full (the internal urinary sphincter automatically opens when your bladder is about 80% full, and you piss yourself).

In very young children with bedwetting problems, the body have trouble producing ADH, and so they do not slow down urine production during the night and they wet themselves in bed.

Also, your bladder is elastic, and can stretch to accommodate more urine when needed. When you get older, your bladder loses elasticity, and you need to get up during the night to pee.