Is the saliva that comes down during sleep pure or impure, especially if it is mixed with blood, and does this necessitate changing the pillow?

Human saliva is pure as long as it is free from blood. A small amount of blood mixed with saliva is excused, and there is no need to purify what it touches on the body or clothes. However, a large amount of blood must be purified. If no bloodstains appear on a white tissue when wiping, then nothing is required.