18wks and waking up at 3am and having trouble falling back asleep..

Any advice? I've been sleepin great up until this week. It's not that I'm uncomfortable or anything.. I just wake up, pee, go back to bed and I start thinking about all sorts of stupid things like 'the wheels on the bus' and other silly things that wearing at work with the kids. I couldn't fall back asleep until like 5am and slept restlessly until 6 and had to get up. Any ideas on how to fall bak asleep when my mind starts racing? Again, no problem falling asleep and it's not that I'm uncomfortable (I have an awesome pregnancy pillow)