Nightmares are dreams of anxiety, fear and horror and range from mild to terrible. Night terrors are on a completely different scale: they are fearsome waking experiences. Although nowadays the word "nightmare" describes "bad dreams" in general, it used to apply only to the suffocating, paralysed state of a night terror.
“The word 'nightmare' is derived from an Anglo-Saxon word (maere) used to describe an evil spirit supposed to descend on sleeping persons, intent on sexual intercourse, or a monster seeming to suffocate the sleeper. [...] Nightmares should be distinguished from night terrors. Unlike nightmares, night terrors are accompanied by powerful physiological effects - heart and respiratory rates shoot up, there is often sweating and sometimes loud screaming. The content of night terrors is often a conviction of suffocation, choking or entrapment. Nightmares, like all dreams, occur during REM sleep, night terrors occur during deep, non-REM sleep.”
Once considered to be the effects of evil spirits (especially the sexual Incubus or Sukkubus demons), the frequency of nightmares varies from person to person, from time to time. Most people find them to be discomforting, worrying and disturbing - but many people simply live with them and don't pay them much attention. Some people find them inspiring for personal drive or artistic output.
“The Old English word mare ('spirit'), from which the latter half of 'nightmare' is derived, eventually came to mean 'succubus'. The connection with the modern notion of a terrifying dream arises from the idea that the nightmare (or night hag, as she was sometimes called) was actually a succubus or an incubus (depending upon the sex of the dreamer) which sat on the breast of the sleeping person and induced such dreams. The etymology of the word has frequently been misunderstood, for the latter half has been taken to refer to the equine 'mare'. This has given rise to images of the nightmare as a demonic horse or horserider.”
"Dictionary of Demons" by Fred Gettings (1988)2
When awaking from nightmares, dreamers are "often aware of the inability to move that characterizes REM sleep"3. Nightmares occur during the same portions of sleep as normal dreams, but are called nightmares due the anxiety and fear contained in the interpretation of the dream by the dreamer. Nightmares typically occur "at the end of a very long REM period. [...] Anxiety nightmares are most frequent when your psychological need for REM sleep is greatest"4.
Dreams of a kind occur throughout all sleep, not just during REM sleep.
"In his book The Sleeping Pill, Ernest Hartmann notes that various illnesses and high fevers often reduce the amount of REM sleep you experience. So do sleeping pills. Thus, anxiety nightmares often occur when you are recovering from sickness, or just after you have stopped taking sleeping pills. Hartmann warns that taking drugs to help you sleep can be dangerous. For once you stop using the drug, your REM sleep "rebounds" and you may have almost constant anxiety nightmares for several nights in a row"4.
Nightmares, occurring during REM sleep, are different to night terrors which occur during non-REM sleep, in Stage 3 and Stage 4 sleep.
Night terrors arise in the early night, from non-REM sleep (also when sleepwalking, sleep talking and sleep screaming occur)5 where the brain's waveforms are long and slow. "They involve brief and less elaborately detailed experiences of entrapment, of being choked or attacked, often with shrieking, sitting-up, or sleep-walking, and tremendous acceleration of the heart. [They become] more frequent when there is greater daytime anxiety; they are frequent among wartime battle evacuees and night terrors are commonly experienced by children aged 10-14"3 and in total about 6% of all people suffer from night terrors6.
McConnel describes a likely Night Terror:
“You begin to sense - deep down inside you - that something has gone very wrong. Slowly, almost dimly, you regain enough consciousness to realize that you are suffocating, that some heavy weight is lying on your chest and crushing your lungs. Suddenly you realize your breathing has almost stopped, and you are dying for air. Terrified, you scream! At once, you seem to awaken. There is this thing hovering over you, crushing the very life out of your lungs. You shout at the thing, but it won't leave you alone.
Despite a strange feeling of paralysis, you start to resist. Your pulse begins to race, your breathing becomes rapid, and you push futilely at the thing that is choking you to death. Your legs tremble, then begin to thrash about under the covers. You sweep the bedclothes aside, stumble to your feet, and flee into the darkness. You run clumsily through the house, trying to get from the thing.
And then, all at once, you find yourself in your living room. The lights come on, the thing instantly retreats to the shadows of your mind, and you are awake. You are safe now, but you are intensely wrought up and disturbed. You shake your head, wondering what has happened to you. You can remember that you were fleeing from the thing that was crushing you. But you have forgotten your scream and talking in your sleep.
The thing dream is a classic example of a night terror. According to Anthony Kales, a scientist at the Penn State Medical School, night terrors occur to but one person in several hundred. However, if someone else in your family has a history of night terrors, you are 10 times more likely to experience night terrors than would ordinarily be the case.”
"Understanding Human Behavior" by James V. McConnel (1986)7
They can be stimulated by anxiety during the day, highly emotional events and by anti-depressant drugs.
In one study "some 58 percent of the subjects [...] could remember part of what they experienced during the attack. The dream episodes included such things as being crushed, being enclosed, being abandoned, choking, dying, falling, and aggressive acts by others. Their sleeptalk typically matched the content of the dream. However, the subjects seldom remembered the extensive sleeptalking and screaming that characteristically appeared at the start of the attack"4.
"Some psychologists believe that sleep paralysis could explain the origin of the alien abduction syndrome"6 and it is easy to see how, also, before the physiological causes of these experiences were known, night terrors were interpreted as being the attacks of evil spirits too. Others have experienced it as attempted possession or as the evil magic of medieval witches along with all manner of other supernatural and paranormal wonders. These interpretations have all been shown to be false by modern science, as all the factors that cause sleep terrors have been found to be solely biochemical in nature. The chemistry precedes the attack.