Isn't it obvious to all of us dick-owners? I can feel that the same muscles involved in boner-making are the same muscles that hold piss back. It feels like those muscles are flexing in the same way for both functions. Morning wood is a side-effect of our bodies trying to stop us from pissing ourselves at night. It just so happens when we sleep, the boner-blood valves are relaxed, like so many other muscles during sleep.