I voted the lone "often."
If I am not having a lot of sex, I tend to have orgasmic dreams. They usually happen right before my period (or before my inactive pills as Loestrin 24 renders me free of periods). I'll be having a very hot dream, become semi-lucid in it, and wake up grinding the air or with my hand up my nightgown.
Sometimes, my noise wakes me up.
I love having these dreams--they make me realize I can probably just fantasize my way to an orgasm. Ah, to be free of all the little worries that inevitably enter the mind and get in the way.