Morning wood is God's way of keeping you from pissing your pants while you're asleep. As your bladder fills up while you're sleeping, it starts to push on your prostate gland, causing the penis to become erect. Since it's damn near impossible to accidentally release the contents of one's bladder while hung, you'll manage to hold it in until you wake up.
After you wake up...well, that's another story. A good exercise in contortion, that.
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