nightclothes vs nightgown

nightclothes

noun
  • Clothing worn while sleeping in bed, such as pyjamas or a nightgown, and, when worn over nightclothes, a robe. 

nightgown

noun
  • A long loose robe worn mainly by women for sleeping in. 

  • A dressing gown. 

How often have the words nightclothes and nightgown occurred in a corpus of books? (source: Google Ngram Viewer )