Nightmares Y ou wake up with your heart pounding, and maybe even with a shout. Isolated nightmares are normal, but when dreams resulting in extreme terror or anxiety recur often they can become a debilitating sleep disorder. A nightmare is a dream that results from a REM ( rapid eye movement) sleep that causes a rise in feelings of strong terror, fear, distress or extreme anxiety. This phenomenon tends to occur in the latter part of the night and oftentimes awakens the sleeper, who is likely to recall the content of the dream.