Dreams are a normal human physiological phenomenon and a subconscious expression of emotions. Due to the various stimuli inside and outside the body and the external stimuli left in the brain during the day, different image activities of different nature will be formed in the brain, that is, different situations of dreams. Some of them are scary and frightening, that is, nightmares. The main reasons for nightmares are as follows: 1. External environmental factors: some factors will directly affect sleep, such as uncomfortable room temperature, light stimulation, sleep posture error, winter cover too thick, etc. caused by physical discomfort; exposure to too many stimulating things during the day causes abnormal excitement in the brain, such as watching horror movies and novels. Ordinary stimulation may lead to dreams, while horror or extremely strong stimulation will lead to nightmares. 2, intrinsic psychological factors: dreams are a special manifestation of intrinsic mental activity in the human body, as is often said, “there is something to think about, there is a dream at night”, work pressure, thinking about the problem will lead to long-term psychological depression, some have experienced major trauma and change is more obvious. Dreams will repeat their worries about the horror of the scene, the formation of nightmares. 3, physiological factors: according to the survey, women have more nightmares than men, consider the reason why women feel more strongly about nightmares, and women are more likely to bring anxiety to sleep; children are more likely to have nightmares than adults, because the brain is not fully developed, emotionally vulnerable to external influences. 4, somatic factors: related research confirms that frequent nightmares can indicate certain diseases, such as a long time dream of a part of the disease related to clinical manifestations, may be the part of health problems, but is not sure, further examination is needed. In summary, environmental, psychological, physical and somatic factors can cause nightmares to appear. Occasional nightmares are normal, but if they affect the daytime work life, the cause should be found and solved in time.