A nightmare is a dream which causes a strong unpleasant emotional response from the sleeper, typically fear or horror, being in situations of extreme danger, or the sensations of pain, falling, drowning or death. Such dreams can be related to physical causes such as a high fever, turned faced down on a pillow during sleep (most often in the case of drowning nightmares), or psychological ones such as psychological trauma or stress in the sleeper's life, or can have no apparent cause. If a person has experienced a psychologically traumatic situation in life—for example, a person who may have been captured and tortured—the experience may come back to haunt them in their nightmares. Sleepers may waken in a state of distress and be unable to get back to sleep for some time. Eating before bed, which triggers an increase in the body's metabolism and brain activity, is another potential stimulus for nightmares.
Occasional nightmares are commonplace, but recurrent nightmares can interfere with sleep and may cause people to seek medical help. A recently proposed treatment consists of imagery rehearsal. This approach appears to reduce the effects of nightmares and other symptoms in acute stress disorder and post-traumatic stress disorder.
Practitioners of lucid dreaming claim that it can help conquer nightmares of this type, rather than of the traditional type (see below).
Nightmare was the original term for the state later known as waking dream (cf. Mary Shelley and Frankenstein's Genesis), and more currently as sleep paralysis, associated with rapid eye movement (REM) sleep. The original definition was codified by Dr Johnson in his A Dictionary of the English Language and was thus understood, among others by Erasmus Darwin and Henry Fuseli, to include a "morbid oppression during sleep, resembling the pressure of weight upon the breast."
Such nightmares were widely considered to be the work of demons and more specifically incubi, which were thought to sit on the chests of sleepers. In Old English the name for these beings was mare or mære (from a proto-Germanic *marōn, related to Old High German and Old Norse mara), hence comes the mare part in nightmare. Etymologically cognate with Anglo-Saxon /mara/ ('incubus') may be Hellenic /Marōn/ (in the Odusseid) and Samskṛta /Māra/ (supernatural antagonist of the Buddha). (This "incubus" is more modernly referred to as sleep paralysis.)
Folk belief in Newfoundland, South Carolina and Georgia describe the negative figure of the Hag who leaves her physical body at night, and sits on the chest of her victim. The victim usually wakes with a feeling of terror, has difficulty breathing because of a perceived heavy invisible weight on his or her chest, and is unable to move i.e., experiences sleep paralysis. This nightmare experience is described as being "hag-ridden" in the Gullah lore. The "Old Hag" was a nightmare spirit in British and also Anglophone North American folklore.
Various forms of magic and spiritual possession were also advanced as causes. In nineteenth century Europe, the vagaries of diet were thought to be responsible. For example, in Charles Dickens's A Christmas Carol, Ebenezer Scrooge attributes the ghost he sees to "... an undigested bit of beef, a blot of mustard, a crumb of cheese, a fragment of an underdone potato..." In a similar vein, the Household Cyclopedia (1881) offers the following advice about nightmares:
Studies of dreams have found that about three quarters of dream content or emotions are negative.
One definition of "nightmare" is a dream which causes one to wake up in the middle of the sleep cycle and experience a negative emotion, such as fear. This type of event on average one per month. They are not common in children under 5, more common in young children (25% experiencing a nightmare at least once per week), most common in adolescents, and less common in adults (dropping in frequency about one-third from age 25 to 55).
Fearfulness in waking life is correlated with the incidence of nightmares.
Scientists speculate that negative dreams are evolutionarily adapting, purging the brain of memories or associations which trigger fear.