My wife has always had problems when sleeping on her back. It makes her more tired and she can often feel pain in the back. So she always sleeps on her side.
Now she is 6 and a half months pregnant, and whenever she tries to sleep on her side, the baby kicks and kicks until she turns onto her back. This goes on all night. Whenever she is able to go to sleep, she unconsciously turns onto her side and is again woken by the baby who keeps kicking until she goes back onto her back.
This means she is more and more tired and uncomfortable every morning, and woke up crying today.
She says it seems like the baby is horizontal, and maybe this is why he doesn't like to be tilted.
Ah yes, I feel your wife's pain... I've been like this for about a month now and am knackered. I bought a Dream Genii pillow on the offchance it might help - and it has. It's not perfect by any means, but it's an awful lot better than how things were. It costs £40. I got mine from Blooming Marvellous but I'm pretty sure you can get them online as well if you have a google.
PS if the baby really is transverse, spending some time on her hands and knees during the day may encourage it to correct itself. Does she feel most of the kicking into her side?
Quite aside from her usual problems sleeping on her back, in pregnancy it's even more uncomfortable as sleeping on her back or her right will squash her vena cava artery. That's why I keep waking up - it feels like I'm suffocating if I roll on my back. In a non-pregnant state that's how I sleep which is why I'm always waking up without the pillow to keep me in place.
Other Mumsnetters may tell you they've never had any vena cava related problems - I think it varies and depends on where the baby is and where your vena cava is.