Nightmare

A nightmare is an unpleasant dream. Nightmares cause strong unpleasant emotional responses from the sleeper, typically fear or horror. The dream may contain situation(s) of danger, discomfort, psychological or physical distress. Such dreams can be related to physical causes such as a high fever; in an uncomfortable or awkward position; stress or post traumatic experiences. Sometimes there may not readily be an explanation. If a person has experienced a psychological trauma, the said experience may haunt them in their nightmares. Sleepers may be 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[1]. The term "nightmare" used to refer to what is now called Sleep Paralysis in the 19th century and earlier.
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.[2] This approach appears to reduce the effects of nightmares and other symptoms in acute stress disorder and post-traumatic stress disorder.[3]