nightmare, disturbing dream that provokes a strong negative emotional reaction. Nightmares differ from other dreams in that they typically cause the sleeper to awaken because of emotional distress. An upsetting dream that does not cause the dreamer to awaken is usually not considered a nightmare. Although the causes of nightmares remain unclear, nightmare occurrence and frequency are associated with various factors, including activity level, age, diet, medication, and physical and mental health. Nightmares typically occur during REM sleep, which is the deepest stage of sleep and is characterized by rapid and irregular eye movement as well as changes in breathing ...(100 of 794 words) Access the full article Help support true facts by becoming a member. Subscribe today!