I found that according to Emedicinehealth.org, frequent nightmares are the result of anxiety disorders. BUT random nightmares for adults are normal. The cause of them can very. We actually don't know why we have nightmares, psychologist have yet to figure out the reason for dreams or nightmares. Nightmares are lengthy dreams with imagery that evokes fear, anxiety, or sadness. They normally occur in the latter part of the night and often awakens the sleeper, who is likely to recall the dream. Like I said isolated nightmares are normal, and can be a normal reaction to stress, anxiety, or trauma. Some clinicians believe they help people work through traumatic events. Another fun fact I learned was from Psychology Today, "Nightmares tend to be more common in girls than boys, usually begin in childhood before age 10." That was interesting because I would think they're more common in boys.