Nocturnal enuresis

a couple reasons
I learned from somewhere last night that there are two reasons people wet the bed, I think from the Wikipedia article. 1. There's a hormone that tells your bladder to hold off on peeing until morning. Vasopressin. Not everyone generates this hormone, and not everyone starts generating this hormone at the same age (but if you still don't have it by the time you're an adult, you're probably not going to get it) 2. I forget, I think it was that most people wake up when their bladder is full.

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 * If you dream that you're peeing in a physical toilet and don't wake up, the solution is to always check to see if you're peeing in a physical toilet, even when you're awake.