Do clothes that a person sleeps in become impure, and therefore prayers are not permissible in them?

The default state for clothing is purity unless one is certain that an impurity has contaminated it. Merely sleeping in them does not make them impure, based on the Prophet, peace and blessings be upon him, who said: "He should not leave (his prayer) until he hears a sound or finds a smell." It is permissible to pray in clean nightclothes that cover the 'awrah, although it is better to wear good clothes in obedience to the Almighty's saying: "O Children of Adam, take your adornment at every mosque." One must avoid clothes that attract attention and cause backbiting.