In ancient times, nightmares were thought to be caused by evil spirits. The word, in fact, derives from a Scandinavian legend in which a "nachtmara" — the "mara" being a female demon — came and sat on the sleeper's chest at night, leaving him with a heavy, suffocating sensation of being awake but paralyzed.
Nightmares have been known to inspire great artists: John Henry Fuseli's 1781 painting "The Nightmare" caused a sensation with its depiction of an incubus crouching on the body of a sleeping woman.
John Newton, composer of the hymn "Amazing Grace," and a slave trader, became an abolitionist after a nightmare in which he saw "all of Europe consumed in a great raging fire" while he was the captain of a slave ship. Robert Louis Stevenson's "Dr. Jekyll and Mr. Hyde" was inspired, in part, by a nightmare.
And Elias Howe, who invented the sewing machine, actually came up with the breakthrough concept of a needle with a hole at the pointed end after he had a nightmare in which jungle warriors brandished spears that had holes in their blades.
In the 19th century, some philosophers blamed nightmares on indigestion. Sigmund Freud actually had some difficulty explaining nightmares as part of his thesis that dreams are the expression of unfulfilled wishes, so he generally ignored them. Later, Carl Gustav Jung described them as part of humankind's "collective unconscious" and said the helplessness we feel in nightmares is a memory of the fears experienced by primitive peoples.
Today, in medical textbooks, nightmares are most commonly defined as a disturbing dream that results in at least a partial awakening. Otherwise, they're just bad dreams.
Sometimes, nightmares are caused or aggravated by drugs, medications or illness. Some foods will trigger more vivid dreams, although researchers haven't pinpointed which ones. Trauma, surgery, a death in the family, crime and accidents also can cause them to proliferate.
But regular, non-trauma-related nightmares, with no recurring theme, occur most frequently in childhood, beginning at around age 2, intensifying around age 5 and then winding down after age 8 or 9. Twenty to 39 percent of all children between 5 and 12 suffer from nightmares, according to one study, while night terrors — a distinct phenomenon that occurs earlier in sleep — affect between 1 and 4 percent of the same age group.
Personality traits are often linked to the frequency of nightmares — or at least the ability to remember them. One study at the University of Toronto found that art majors were three times as likely to have nightmares as those majoring in physical education.
While culture may play a role here — a macho athlete might have more difficulty admitting he had a weird dream — "the bottom line is that people who have a good component of openness to the external world, who are more in touch with their own sense of self and emotions, are perhaps more able to remember their dreams," said Anne Germain, a researcher at the Western Psychiatric Institute and Clinic in Oakland who studies patients with recurring nightmares.
Some sleep researchers consider the occasional nightmare a natural response to stress, the body's way of practicing its "fight or flee" response, a way for us to work through aggressive feelings in a safe way, since the body's muscles are essentially paralyzed during rapid eye movement, REM, sleep.
But many psychologists — some of them trained in the writings of Freud and Jung — believe that nightmares, like all dreams, carry much more meaning within them, and that they are the psyche's way of alerting us that something is wrong.
"These stories that come to the surface, spontaneous, unbidden, are from the so-called part of the psyche beneath the surface, like the iceberg in the movie 'Titanic,' " said Thomas W. Sheridan, a clinical psychologist who serves on the medical staff at Allegheny General Hospital's Department of Psychiatry in Pennsylvania. "But medicine is not very good at attending to this."
"The trick about nightmares is to ask ourselves: Why is this happening now in my life?" said Charles McPhee, an author of two books on dreams who calls himself "The Dream Doctor" on a nationally syndicated radio program. "Dreams are mirrors of our waking life. Nightmares are a way of alerting us to what is unresolved in our lives."