Why does a hand in warm water while asleep cause bed wetting?

To: Matthew Alice:

I've never been much of a camper, but I've heard that as a trick, the person that falls asleep first has his hand placed in a bucket of warm water, which causes him to have an accident in his sleeping bag. Why does that happen? It seems to me that your body, if overcome by the urge to pee, would wake you up. I've had dreams in which I was peeing and woke up a few minutes later, surprised that I didn't really go in my bed. Why would warm water change that?

-- I.P. Freely, the net

We now know way more than we wanted to, I.P., but I appreciate your logic. You don't need a Coleman stove and a pup tent to make the trick work. You don't even need warm water. Any nurse will tell you it's one of the ways they get patients to start peeing if they're having problems after surgery or the removal of a catheter. It apparently works by suggestion, the same way running or dripping water might make someone have to go, since there's no direct sensory link between your hand and your bladder. The trick won't make everyone pee in a bag, maybe just heavy sleepers or people with iffy control. More likely the victim will wake up and be really annoyed. Maybe even pissed.

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