Asleep

Which is preferred?
She was fast asleep!
or
She was sound asleep!
We have divided opinions here and would appreciate any and all input.

I hadn't thought of this before but I think, "fast asleep" is that she's fully asleep.

"Sound asleep" implies she's in a deep sleep.

The difference if there is any is very slight so I'd think either way of saying it would do.

I haven't given it any thought until now either. I am not sure, but I think I probably use them very much interchangeably - no doubt I will now be shown to be unspeakably sloppy!