Dude, this is probably the funniest topic I've read on these forums.. I think I have the answer to the morning tree trunk... when you sleep, your body goes "offline"... when you start to wake up, your body realizes this before your conscience can so your body a sort of "self test" and kind of reboots itself. The blood flow is actually sent to ever part of your body when you wake up... The reason you sometimes don't wake up with a wood all depends on how you sleep. If you sleep in a position where blood flow is limited to the tree stump, then the morning harvest will be HUGE... but if your flow is decent in your sleep, then you might not experience wood at all or just a semi...
Of course sleeping with a naked beautiful woman can help get you horny when you wake up.. it must be the smell of sex in the air.
(Homer voice) MMMMM sex... uuggghhh. (drool)