Sleeping w/pillow between knees...Why?

[tongue_tied]

Anyway, I've been wondering what problem(s) the pillow-between-the-knees 'fixes' or at least addresses. Because, ideally, I'd rather fix the problem itself than continue to *have to* use a pillow at night. I'm thinking that if I can get to the root of whatever is too tight, misaligned or whatever, I can get rid of both the back/hip pain AND get a decent night's sleep again.

Thoughts?

The pillow between the knees fixes your hip alignment. I don't normally sleep with a pillow between my knees, but I know that some nights, there's a hip pain and I read somewhere that this "fix" helps.