Sometimes I worry about things and can't sleep -- who doesn't?  But often I can't fall asleep because my mind keeps running on, continuing discussions I've had, or thinking about things I've read.  I have conversations with friends and relatives.  At some point I drift off and the conversations become dreams.
Oh, and I sleep in the buff, too.  I'd rather add another blanket or turn up the electric blanket a notch than wear night clothes.