My OB's office has Unisom on the list of approved medications for sleeping trouble. Are you waking frequently or just not able to sleep? Try having ten or fifteen minutes after putting the kids to bed to unwind and get your brain ready for sleep. By that point in pregnancy I often slept much better curled around a body pillow so my belly was supported and sometimes I had pillows propped so my torso was higher than normal and I could breath more easily.
Oh, and I agree that this is a question for your doctor, but one thing my doctor's office has that I love is a list of over-the-counter pregnancy-safe medications, the dose, and what to take it for. I keep that in my medicine cabinet so I don't have to call the answering service in the middle of the night or on a weekend. See if your doctor has something like that you could take home.