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Even though there is no way of knowing when a nightmare has a natural cause or a demonic one. We can only suspect that it has a demonic cause when there are other indications that point to this.

There are cases where no psychiatrist has been able to find a reasonable explanation, either conscious or subconscious, for a normal person to suffer every night for a month or more with terrible nightmares that cause him or her to awake screaming and covered in sweat. These periods of very intense nightmares are sometimes connected to things such as having taken part in an occult rite or having begun a more intense spiritual life. The best advice one can give someone in this situation is to use holy water and ask God for protection and deliverance from any demonic influence during the night before he goes to sleep.

If such actions cause the nightmares to stop, this would confirm that they were demonic in origin. 1

The official criteria define a nightmare as a frightening dream that occurs in REM sleep, causes the dreamer to awaken, and creates emotional distress. Many scientists who study nightmares (me among them) argue that that official criteria need revision. Many nightmares do not cause you to awaken. Another distinguishing mark of the nightmare — besides the level of terror involved — is its content. Nightmares very often involve supernatural characters that attack or target the dreamer in some way. I mean monsters, creatures, demons, spirits, unusual animals, and the like. From a cognitive point of view, one interesting aspect of the presence of the supernatural being in a nightmare is that the dreamer cannot read the mind of the supernatural being. All we can do is understand that the monster’s intentions are malevolent.

Nightmares also often involve the dreamer, or self. Interestingly, the self responds to the monster with a wide range of feeling, from terror to awe and fascination. The self escapes unscathed only if it refuses to look at or speak to or in any way engage the monster. When the self engages the monster, all kinds of ill effects ensue, including, in ancestral cultures, demonic or spirit possession. There is a danger involved in the encounter with spirit beings; you may not psychically survive the encounter. Instead, the malevolent spirit will take up residence in your consciousness and control your actions. You become possessed. It is an interesting clinical fact that, even today, most cases of involuntary spirit possession across the world occur overnight. The person wakes up possessed. Traditional cultures have developed ways to identify the demon-possessed people. They are usually self-destructive, they have chronic physical pains and physical distress, they are irreverent toward the culture’s religious rituals, they are restless, and they have recurrent nightmares. 2

1 https://spiritualdirection.com/2013/08/22/can-demons-cause-nightmares

Fr. Jose Antonio Fortea 

Father José Antonio Fortea is not only an exorcist, but also a writer, and parish priest. He once thought he would lead what he has termed ordinary life as an attorney in Madrid, much as his father did before him, but sensed instead a vocation to the priesthood in his adolescent years. A theology graduate of Navarre University in Spain, Father Fortea wrote a thesis there on exorcism. He has been a practicing exorcist for several decades.

2  https://www.bu.edu/bostonia/spring09/nightmares/index.shtml

Dr. Patrick McNamara

Neurocognitive scientist and School of Medicine associate professor of neurology and psychiatry spent ten years researching and writing about them. The result of his efforts, Nightmares: The Science and Solution of Those Frightening Visions During Sleep, was published last year by Praeger. McNamara’s book covers a lot of ground, from cultural interpretations to biology to horror films inspired by our most disturbing dreams.