Before editing.

The current usage of the term nightmare refers to a dream which causes the sleeper a strong unpleasant emotional response. Nightmares typically feature fear or horror, and/or the sensations of pain, falling, drowning or death. They can be related to physiological causes, such as a high fever; psychological ones, such as unusual trauma or stress in the sleeper's life; or commonly for no apparent cause. Nightmares can be so stressful as to suddenly wake the sufferer in a state of distress, which may prevent falling back to sleep for some time.

Edited.

The typical use of the word nightmare refers to a dream which cause the sleeper strong unpleasant emotional response. Nightmares feature fear, horror, sensations of pain, falling, drowning or death. They can be related to physiological causes, like high fever. Psychological ones, like unusual trauma or stress in sleeper's life or commonly no apparent cause. Nightmares is stressful as to suddenly wakes the sufferer in a state of distress, which prevents falling back asleep for some time.