A nightmare is a bad dream that causes you to wake from your sleep. Like other dreams, nightmares tend to reflect your daytime concerns. They occur more often when life events cause you to feel anxious or stressed. Nightmares often occur as a result of a traumatic event. In the dream you may experience the traumatic event all over again. It is unclear why nightmares occur. Theories suggest that dreams and nightmares help your brain process both old memories and new information. The brain also may use dreams and nightmares to help you adapt to events in your life.