I had the same problem all through my pregancy.....incredible tiredness, but I found it very tricky to "switch off" and relax my mind and body enough to go to sleep.

My DH bought me a CD of relaxing "Sleep" music and even this didn't work, but as we had the CD player set up in our bedroom I tired other CD's to help me wind-down. This sounds a littel extreme, but bear with me! The one that absolutely worked for me was a hypnotherapy CD that I had used years before (for giving up smoking!). Call me mad, but the ladies voice and the breathing excercises & the "dolphin like" sounds and music worked in minutes.

I still use theis CD now and am usually fast asleep within a few minutes...the habit and the breathing etc really work.

So my suggestion would be to get a hypnotherapy style CD and try it for a week.

Maybe look for a self hypnosis CD (they would all be available from Amazon, or Angus and Robertson). There is one called "Mummy Hypno" which is for relaxing pregnant woment and helping you to relax and enjoy the pregnancy and to relax enough to actually sleep.

Best of luck,