One of my theories for this (I think my Mum told it to me when I was pg) is that your bub inside you has a metabolism of its own, which is always going at a super fast rate what with all of the growing and developing thats going on, and maybe its possible that it somehow interferes with our own natural sleep patterns etc?
This used to make sense to me as I would lie awake at night when pg, feeling like there was this little annoying 'buzzing' feeling inside me that just would not let me sleep!
Or that theory could be a load of baloney too... LOL 8-[

Sorry, but I actually dont have any suggestions except for maybe doing meditation or relaxation excercises before bedtime...