I love erotic dreams, especially if they're lucid. Dreaming can be one cause of your erection. The connection between having to pee and having and erection is one lots of men experience, too. SInce peeing uses many of the same muscles (and can sometimes be almost as gratifying!!) as ejaculating, when you get the urge to pee, that could provide the spark that begins an erotic dream and leads to a hard-on.

If the waking up is bothering you, try not drinking anything for a coupe for hours prior to bed but make sure you;re hydrated during the day.

Me? I just enjoy it!

I have this at night.  According to my urologist, it's from squeezing the muscles tightly during sleep to retain the urine.  Pee, go back to bed.  Try cutting down on liquids before sleeping to minimize it.
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I've had this my whole entire life. No problem, just a solution to not wetting the bed while getting a little more sleep.