I find the painful and uncomfortable most of the time. Really annoying when you want to spoon and the little guy is pointing straight into her back. For the record, I can urinate while hard. Ask a couple of my buddies and they say they all can too. Is this uncommon or we just freaks of nature?
Actually, it does happen all the time – to teenagers. Don’t you remember those embarrassing “I cant’ stand up yet, so I’m going to fumble with my books until this goes away” erections from high school? Even during moments that are completely free of erotic notions (rare though they are), they pop up, and they won’t go away until you relax a little, but you can’t relax if you need to urinate as well. You just sit there, clenching your teeth, waiting for a miracle (unless yu shuffle off trying to hide your loins with a notebook).
A full bladder does not cause an erection. People associate it with a morning erection because it’s typical to have them both at the same time, but that’s coincidence, not causation. The reason for the full bladder is going for hours without urinating. The reason for the morning erection is the REM cycle, as mentioned above. One does not cause the other.
They talk about a few studies to this effect in Our Dreaming Mind by Robert deCastle, and it happens to women too- blood flow to the genitals waxes and wans according to the REM cycle. The implements used to record the pertainent information that allowed them to conclude this sound like they’d make sleep impossible, but… 
I’m not sure if you can find them online, but the studies were:
" Cycle of Penile Erections Sychronous with dreaming (rem) sleep" by C. Fisher, J. Gross and J. Zuch
and
" Vaginal pulse amplitude response patterns during erotic conditions and sleep" by G. Rogers (ODM doesn’t go into details on the “erotic conditions” part of the study.)
I’ve read that men with erectile dysfunction, that is possibly philological, could try the following. Take a roll of postage stamps, circle your penis while it is flaccid, and connect the stamps at that point. If, in the morning, the circle you made has been broken, you don’t have a blood flow problem, and perhaps you should seek out other reasons for not being able to achieve an erection.
(there’s a Johnny Cash song in here somewhere) 
Well, I can’t speak for all old guys, but yes.
My psychology teacher was discussing this in class. Every once in a while I see someone post something about what a whacky college professor said, and here’s another:
Erections during REM sleep may have evolved to act as a sort of kick-stand so we don’t roll over and possibly damage our testicles while asleep (or something along those lines… I distinctly remember him saying “kickstand”)
OMG, you just described my high school experience. I thought I was the only one. My soldier saluted at 10:30am without fail, every day. This was about 5 minutes before class let out, so I’d walk to my locker with my binder or books held low and it front. It was a pain.
Vlad/Igor
Ring of Stamps?
A Boy Named Glue?
Wait, wait, I just got it (apologies for being so slow):
Will the Circle Be Unbroken
I’m proud to have reached half a kilo-post with two earnest, irreverant musings about the great Johnny Cash.
Mods, move if you see fit.
(Now I’m just running up the score…)
Almost daily morning, er, stiffness here. No idea why. I, too, have associated it with the urgent need to take a leak, which is inhibited somewhat (though certainly not prohibited) by penile tumescence.
I have to say, some of my AM woodies are throbbing erections of the highest order of turgidity. I feel like I could lie belley-down, and with a kick of my legs, spin about in place. Almost painful (not helped by the full bladder thing). And hardly ever because I’m rearin’ to go. Mostly I’m needin’ to pee.
That would be a “humby peeover”, as I recall from The Meaning of Liff.
I just read (this months Glamour Magazine?) that they are not associated with any desire for sex. The man in the article said he doesnt want sex when he wakes up that way, that although erected it is sort of tender, and that it is better to wait until later, after breakfeast, etc. if you want to do it in the morning.
Perfect answer. May I follow through?
It isn’t hard (heh) to pee when hard. What’s hard is getting it into a toilet bowl. When we go to the cabin it’s easier and more natural to “hit the air” as we call it. Therefore the men go to the edges of the woods (heh, again) and just let loose.
The only physiological fact I’ve seen is that urine can’t be passed along with semen during ejaculation.
Hope that helps a little.
Although the bladder is closed off, whatever (drops of) urine might be (remaining) in the uretha tube will be passed/flushed along.
Get your angles right and you can shower at the same time.
I’d say quite the reverse if anything; insensitive rather than tender. Which is a shame as it’s often the hardest kind of erection I get (mind you, when Mrs M is remotely interested at that hour of the day you can mark a commemorative day on the calendar).
Real men just do a handstand and let fly while doing inverted pushups.
Me too, but it does seem such a shame to waste a perfectly good hard-on.