I am looking for a word to describe a person that can fall asleep 'as their head hits the pillow'.
To add context: my wife falls asleep seconds after she decides that she wants to sleep. Not just in bed, but on the sofa, on the train - almost anywhere where there is a relatively comfortable place to sleep. Even on a moving escalator. She is a person who I consider to be happy and at peace with herself. I mention this as I am not looking for a term to describe someone with a medical condition with no control over when they sleep.