Published: 14 May, 2008

How is a nightmare different from a scary dream?

Why do we have nightmares/night terrors? How long do they normally last?

Your question treats "nightmare" and "night terror" as synonyms. This is exactly right, if we understand "nightmare" in its original meaning: "a sensation in sleep resembling the pressure of a weight on the chest . . . seeming to be that of some hideous monster or phantom, interrupting . . . sleep with a violent struggle." Today, "nightmare" is used interchangeably with "frightening dream." To make sure that I answer your question, I'll briefly describe the frightening dream, and then the night terror.

A frightening dream originates in REM sleep: sleep with rapid eye movements and waking-like brain activity. The frightening dream is story-like, and is often remembered after waking. For example:

I just happen to live across the street from Freddy Krueger's old house (Nightmare on Elm Street) . . . My friends and I go into Freddy's house. There are many different little narrow hallways and rooms . . . We explore the basement. Every room we look into strikes terror in me . . .

Like many such dreams, this one builds to a scary climax, but ends inconclusively:

Suddenly Freddy tries to push his way in. I try to keep the door shut by pushing all of my weight against it.

The frightened dreamer may escape, willy-nilly, or even by force of will:

I fly up to the ceiling and squawk like a chicken and flap my arms at him. He cannot scare me!

Oddly the REM sleep dreamer--whether terrified or defiant--does not get very excited. Heart rate and breathing rate do not rise significantly. The dreamer may sweat, but not much.

In high contrast, the night terror is a physical and emotional cataclysm. It is often announced by a scream. The heart beats violently. Breathing is rapid. Sweating may be profuse. Yet night terror is rarely remembered. The occasional account is vague but formulaic: The victim is helpless, and is about to be lifeless. Somehow locked in mortal struggle, he or she will be crushed, or strangled, or dropped from a great height, or drowned, or incinerated, or the like. We are relieved to wake from a frightening dream, but in night terror we are already half-awake, and yet we cannot fully wake. We ignore consolation. We may lash out at intervention.

The night terror originates in slow wave sleep, in which huge populations of the brain's neurons fire in synchrony, then fall silent in synchrony, then fire together again, in a repeating pattern. Arguably this pattern of brain activity compresses and magnifies experience of all sorts, including the experience of fear. Fortunately slow wave sleep -- unlike REM sleep -- blocks awareness, and most of us experience nothing at all in this sleep state.

In some people, and particularly in some children, two factors predispose to night terror. First, rhythmic brain activity in slow wave sleep may exceed its usual intensity. Second, the natural barrier between slow wave sleep and wakefulness may be immature, or otherwise vulnerable. With these conditions in place even a neutral stimulus can partly waken, and completely terrorize, the sleeper. Typically the experience of night terror decreases in young adulthood, as the boundaries between sleep and waking mature fully.