Merry Christmas！And I've got a question to ask on this wonderful holiday.
It's an example sentence under the entry "sink" in New Oxford Dictionary:
(of a person) lower oneself or drop gently: She sank back onto her pillow.
I want to confirm if the word "back" in the sentence is used as an adverb, which means returning to her early lying position. Is it possible that it is a noun which means "her back"? Then the sentence would mean she lay in her bed with her face up.
I would prefer the former explanation, but still can't be very sure to expell the later.