All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The incubus is the nightmare that comes while one is sleeping. Some contemporary physicians call it "sleep paralysis", and former and contemporary physicians have spoken about it. They said that it is due to physical causes in most cases and it might be due to other reasons. Ibn Seena (Avicenna) said in his book Al-Qaanoon (the Law): "Chapter on Incubus: It is called the suffocator and it can be called in Arabic Al-Jaathoom, ..... it is a disorder in which a person feels when he is in the first stage of sleep that a heavy imaginary thing falls on him, squeezes him and constricts his breathing, so one loses his ability to speak or move. He almost suffocates due to the blocking of the pores of his body, and when it leaves the person, he suddenly becomes attentive, and it is an introduction to one of the three illnesses: either epilepsy or stroke, or mania. That is if it was due to congested matters and not due to other non-physical reasons..." [End quote]
It may also be that the Jaathoom is because of the jinn either by possessing the person or by taking control over him through imaginations and nightmares. Abu Qataadah said that the Prophet said: "The good dream is from Allaah while the bad dream is from the devil. So, if any of you has a dream that he fears, he should spit on his left and seek refuge with Allaah from its evil, for it will not cause him any harm." [Al-Bukhari and Muslim]
Allaah Knows best.