Does saliva that comes out of the mouth during sleep, which may wet the hair or clothes, affect the validity of prayer if it is not washed off?

The saliva that comes out of a sleeping person's mouth does not require washing, because it is pure. However, if one is certain that it originated from the stomach, then it is impure. But if it is widespread and difficult to avoid, it is excused due to hardship. It should be removed with water or something else if it forms a layer that prevents water or dust from reaching the skin during ablution (wudu') or dry ablution (tayammum).