My girl never howled in her sleep, she only ever made "splashing" yelps.
I never heard her howl at all.
One day, my mate came round, we packed rucksack and outdoor jackets, loaded the car - and drove off without her. Apparently she howled and howled at the gate after we'd gone.
We were going skiing that day and didn't think she'd cope with the skilifts, or keep up with us going down.
When I was told, I felt so guilty, I've never been skiing since.