I never say PJs, primarily because I really like the word pajamas. Always have. It's the same word in both of my languages: pidzama/pajamas.
Oxford English Dictionary:
pyjamas | pajamas, n.
Etymology: < Urdu pāy-jāma, pā-jāma and its etymon Persian pāy-jāma, pā-jāma, singular noun < Persian pāy, pā foot, leg + jāma clothing, garment (see jama n.1)
Originally: loose trousers, usually of silk or cotton, tied round the waist, and worn by both sexes in some Asian and Middle Eastern countries. Subsequently also: nightclothes consisting of loose trousers or shorts and a jacket or other top (now the principal use). The loose trousers were adopted by Europeans living in Eastern countries, esp. for night wear, and the word came to be applied outside Asia (originally in trade use) to a sleeping suit of loose trousers and jacket.
So, to be technical, I usually sleep in pajama bottoms and a t-shirt.
My dream job would be to test pajamas, bed linen, and mattresses/bed for the manufacturers. I excel at sleeping.