I am a habitual victim of the morning wood. Pretty much every morning it rises before I do. Very annoying because the first thing I want to do upon waking is take a piss and my cock curves upward making it virtually impossible to piss in the toilet.

It sounds crude but I usually just piss in the shower and wash it all down. I remember reading somewhere that you get an erection during sleep due to deep REM sleep (ie dream sleep). I do have very vivid dreams practically every night so that could be the reason.