An item of clothing often made from cotton or another absorbent material, in the form of a long open robe with a belt to tie it around the middle and fasten it securely; often worn over pyjamas.
nightrobe
noun
A robe to be worn at night; a nightgown.
How often have the words dressing gown and nightrobe occurred in a corpus of books? (source: Google Ngram Viewer )