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An entry from Wikipedia
A nightmare is a dream which causes a strong unpleasant emotional response from the sleeper, typically fear or horror, or the sensations of pain, falling, drowning or death. Such dreams can be related to physical causes such as a high fever, 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.
One nightmare that I can recall is where I am standing on a deserted runway, the sun was down and a slight wind was blowing. Then right in front of me, a huge jet plane lands and screams towards me. The sound from the jet engine is deafening and I could not move my limbs. Just before it hits me, the jet plane turns and stops. I was shaken but I still could not move.
I woke up and truly enough, could not go to sleep for hours.