Sleepwear was clothing designed to be worn while sleeping.
Louise, Lewis's wife, and their children, Loretta and Harold, wore sleepwear (pajamas or a robe) in 1985A. Lewis himself slept in the underclothes he had worn during the day, which consisted of black-spotted boxer shorts and an undershirt. In the early hours of October 26, the Lewis family was wearing this sleepwear, when Loretta screamed for help as Marty McFly stumbled through the window into her room.
Marty himself tended to sleep without changing out of the clothes he wore during the day, for which his older brother Dave made fun of him. However, while he was in 1885, Marty slept in long underwear (also called "long johns").