I've been getting this too.
Just starting the 3rd Trimester and for the last fortnight I've been waking up in the middle of the night and not being able to get off again. Mix it all in with really vivid dreams. I gather it's all normal - but still aggrevating. I used to read or do a puzzle - but I'd end up getting to an exciting bit and not want to stop thus defeating the object.
If you can't catch up on sleep at other times, I was told this trick by a sleep counsellor. After 10mins of laying awake, get up and do something so mind numbingly dull for 20mins - chopping veg for tomorrow's dinner, washing the kitchen floor really meticulously the hard way, polishing door knobs, waxing wooden floors, ironing your t-towels, bull-polish shoes. Don't put on any music or the telly or do anything remotely interesting/stimulating or which requires concentration. After your 20mins, try going back to bed. Give it another 10mins - if no joy, go back to your deadly dull job. Repeat.
It might take a few goes over a few nights, but your mind will quickly train itself that its choice is getting really, really bored or going back to sleep. Just think of it as the grown up version of what Nanny911 does when repeatedly returning kids to their bed.
It's much much better than laying there stewing. The structured time limits are an important part of the exercise. It doesn't matter if you've finished the job in hand or tidied up. Just stop and go straight back to bed at the end of the time.
And if it doesn't work - well - at least you'll've got a few chores done.
Only other thing I've found to help is a relaxation track on my i-pod by the bed.
Hope some of this helps. Sweet dreams!