I know there are alot of posts about inception but I think this could work.
If you have seen inception then you know they play music when it is time to wake up, and it goes through into the dream.

So what if you played gentle music at a low volume for like 20 minutes when you would be in rem? And everytime you hear that song RC.

Play it quietly so you just hear it and it doesn't wake you up.

I have heard that you can only hear external things at a certain part if a dream? Is it at the start, the end, or all the way through?

And if it wakes you up it would do it gently because it's quiet so you could Deild.