Hi
I've had both hips done. On both occasions I was told to sleep on my back for the first six weeks - I usually sleep on my side. After the first BHR I wasn't looking forward to going through it again. However, the 2nd time seemed easier.
Both times I wedged pillows either side of me to prevent me turning over. If you haven't done that it is possible that your mind is thinking about the possibility of you rolling over and that's what is keeping you awake.
My understanding is that lying on your back reduces the possibility of dislocating the hip when you turn over. That was something I definitely didn't want to do and was prepared to put up with the inconvenience of the six weeks on my back.
After the six weeks was up I got myself onto my side with a pillow between my legs to support the operated hip (it was on top) and another behind my back to stop me rolling over. I kept that up for a few weeks.
Having said all that, I don't have any idea what the effects of sleeping on your front might be !
Best wishes
Ed