"I would wake up so terrified that I was afraid to go back to sleep ," Levy says. "And the bad feelings were hard to shake. I would continue to feel frightened throughout the next day."

In one recurrent nightmare, Levy was trapped in a concentration camp, facing death. In another, she was drowning in deep water. At their worst, the nightmares occurred on an almost weekly basis, leaving her jittery and desperately fatigued.

Yael Levy recalls having chronic nightmares as far back as elementary school, when she was living in Israel. The grandchild of Holocaust survivors, she says her dreams were filled with images of suffering and death.

Chronic Nightmare or Bad Dream?

There's nothing unusual about having an occasional nightmare (which sleep experts define simply as a bad dream that causes the sleeper to wake up). But up to 8% of the adult population suffers from chronic nightmares, waking in terror at least once a week.

Sometimes the nightmares are so frequent and so upsetting that they make sound sleep all but impossible, setting the stage for fatigue and emotional problems like anxiety and depression.

Nightmares vary widely in their themes and specific content -- experts say they can be "about" anything -- but all cause fear, sadness, anger, shame, or another negative emotion. They occur during REM sleep, typically in the latter part of the night. Though more common in children and adolescents, they also strike in adulthood.

In many cases, chronic nightmares are triggered by psychological stress -- such as that stemming from posttraumatic stress disorder, a severe anxiety disorder that strikes people who have been exposed to or witnessed combat, violent assaults, accidents, natural disasters, and other terrifying ordeals.

Other causes of chronic nightmares include alcohol abuse, the use of certain medications, and sleep disorders, including the disordered breathing condition known as sleep apnea.